In Ressentiment, the game containing the original AI girl of which all others are simplified copies is found lying under a rack of disks in an ordinary game shop.
In One Piece, one of Zoro's swords, which is cursed, but immensely powerful if it can be controlled, was found in the armory equivalent of a bargain bin, precisely because it was cursed.
The rare medal used by Metabee in Medabots was originally found by the riverbank. The first episode opens with the Phantom Renegade stealing the medal and evading the Rubberrobos, then accidentally dropping it into the river.
In the beginning of Magical Stage Fancy Lala, Miho first finds Pigu and Mogu at a toy store when they attach themselves to her book bag. The mysterious man who pops up throughout the series pays for them because the store clerk thinks she's stealing them.
In Yu-Gi-Oh 5Ds, Godwin had three of the cards for the eponymous 5 Dragons, and quietly released them to the public to let destiny take its course. Aki is given her Plant-based deck as a birthday present by her father. Besides the more often used effect, Black Rose Dragon also has an effect which works in tandem with Plants, so it seems likely that she got it then. Yusei and Jack probably would have found Stardust Dragon and Red Dragon Archfiend, respectively, in the trash, as the two lived in the Satellite slums and had to assemble their decks from discarded cards. They also might have gained their dragons during their 'Team Satisfaction' days, where they engaged in card-based turf wars until they had beaten everybody in Satellite, with the dragons perhaps taken as prizes. The cards wouldn't have been easily detected by others since they wouldn't be particularly special to anyone who doesn't have a Sign of the Crimson Dragon.
In Soukou No Strain, Sara finds a discarded doll in the scrapyard area of a ship while looking for a pendant someone stole from her locker. The doll is pretty dirty and seemingly abandoned, and when Sara sees EMLY imprinted on the back of the doll, she gives her the name Emily, and grows attached to the doll. Emily also allows her to control a Strain, a mecha that needs a specialized MacGuffin in order to activate (Sara's was destroyed in the first episode, so she was unable to control one again until this point).
Happens at the end of the second Cerebus book, High Society, where it turns out the priceless bird statue that Cerebus could have used earlier to unite the Church factions was given to him with a bunch of other random trash from a hobo who seemed to have Cerebus confused with someone else about a third of the way through the book. He destroys it, since he no longer has the political capital to make use of it himself. And it turns out to be a fake anyway.
In the The Adventures of Tintin book The Secret of the Unicorn, Tintin buys a model ship from a street vendor to give to Captain Haddock as a gift. It turns out the ship has a scroll concealed in the mast which, when combined with two others from identical ships, leads to a fortune in gold and jewels.
Double Subversion in Archie Comics. Jughead finds an old violin in the trash bin outside the pawnshop. When a suspicious man tries to steal it from him, Archie believes that it's a Stradivarius violin. He and Jughead head off to a music shop to get it appraised, only to learn that the violin is no Stradivarius, just a complete piece of junk. On the other hand, it really is a lot more valuable than it seems, and not in the way that the gang thought it would be: the violin's bow is the cache for stolen diamonds.
In The DCU, the Guardian's origin involves him finding a seemingly indestructable shield in a costume shop.
Garfield's teddy bear Pookie may not a be a "grail" to anyone but him (although to Garfield, he's a "security blanket" and a source of unconditional love) but Garfield found him just lying around in a dresser drawer.
The original origin story of the original Green Lantern, Alan Scott, had him find an old railroad lantern while travelling by train (it was 1940, people did that), and only realize it was a powerful artifact when it kept him from being killed. Later tales established that the lantern had been on Earth for centuries, occasionally doing something odd, but mostly just being a lantern.
Woody in Toy Story 2: Al discovers him in a yard sale (accidentally; Mom doesn't think Woody should be thrown out) and rejoices that he's finally found his Holy Grail, as he's been collecting a full set to restore and sell to a museum.
In The Adventures of Tintin (Secret of the Unicorn), Tintin buys a model of a three-masted sailing ship, the Unicorn, for a pound but it is later discovered to contain a parchment scroll which has a map of buried treasure
Films — Live Action
Merlin's Shop of Mystical Wonders is the embodiment of this trope, and it even gets more trope-y when the Lord of Evil is in the form of a monkey cymbalist that's stolen and sold to a ratty toy shop.
In The Forbidden Kingdom, the golden staff of the legendary Monkey King is found in a Chinatown pawn shop specializing in Wuxia DVD's.
The original ending for Monty Python and the Holy Grail would've involved King Arthur finding the grail in... Harrod's. "All Things for All People, Everywhere" indeed. Unfortunately, the Pythons ran out of money and had to end it with everyone getting arrested.
David was about to sell the morphing cube online before the Animorphs caught wind of it. Worse, he was about to unwittingly sell it to Visser Three, probably for way, way less than it would have been worth even if the Visser didn't intend on infesting David the moment he sold it.
In another installment, the Helmacrons' spaceship gets lost in a toy store and Cassie has to find it.
Robert Louis Stevenson's short story The Bottle Imp: A bottle that can grant wishes has a price of only $80 because it can only be sold for less than its previous sale price. Why would someone want to sell such a bottle? Because if a person dies while owning it, his soul automatically goes to Hell.
In The Hobbit (written first), Bilbo finds the ring on the ground, after Gollum misplaced it. So you have an ordinary ring owned by some deformed hobbit. (As the narration says, "It was a turning point in his career, but he did not know it. He put the ring in his pocket almost without thinking; certainly it did not seem of any particular use at the moment.") Then Bilbo discovers it can make the wearer invisible.
Then in The Lord of the Rings, we discover that Gollum's friend first found the ring many years ago just lying at the bottom of a river. So it seems like Bilbo has an "ordinary" ring of invisibility — until we learn it is far more than that.
Gandalf thinks that Bilbo doesn't know the true value of the mithril mail shirt that Thorin give him, and that he left it as a mathom in the Shire. The truth is that Bilbo really knows its true value and gave it to Frodo.
To a lesser extent, there is the small dagger that Bilbo finds in the troll's lair. He doesn't think much of it at the time; it's a useful blade to someone his size, but doesn't seem to be anything else. As it turns out, it's an elven blade forged in Gondolin during the First Age, just like Orcrist and Glamdring. (Bilbo later gives it the name "Sting".)
The eponymous book in The Neverending Story, found in (or rather, stolen from) an unsuccessful antique bookstore.
Chivalry, a short story by Neil Gaiman, features an old woman who buys the Holy Grail in a secondhand shop. She has a bunch of items like it, and uses them to decorate her house. After she buys it, Sir Galahad of the Round Table stops by and offers her such gifts as the Philosophers' Stone, an Apple of the Hesperides, and a phoenix egg, in return for giving up the Grail. Giving it to him, she goes to the store a second time. She considers, for a moment, buying what is heavily implied to be the lamp from the tale of Aladdin... before realizing she has nowhere left to put it.
Abdullah of Castle in the Air buys a plot-important flying carpet from a tattered, dirty traveling carpet salesman.
And again in Harry Potter and the Half-Blood Prince, Harry hides the Prince's potions book in a room where lots of other students (and teachers!) have hidden things they didn't want found over the years. One, which Harry uses to mark the place where he hid the book, is the lost diadem of Rowena Ravenclaw, and another of Voldemort's Horcruxes.
MirroringThe Lord of the Rings, the eponymous Sword Of Shannara is found in the last place you'd expect it — in a bunch of junk that a looter picked up off a battlefield. Despite every legend about it saying it was embedded in a block of "Tre-stone" in the druids' castle. Subverted, though, when the looter knows that the battered, cheap sword with the gold paint peeling off it is the most valuable weapon in the world, even though the heroes don't, and refuses to let go of it.
In The Serpent's Egg trilogy, Typhoon gives a busted up crown to Penelope as a reward for watching a very much alive and ready to hatch dragon egg, which she was told was a rock. It was far bigger than her. The dinky little thing turned out to be the Crown they had been looking for the entire time, but didn't realize it till she placed it on the head... of the enemy. It proceeded to kill the evil. Then, she put it on the Elf Prince's head, to no real effect. Notably, she tried to steal from Typhoon earlier, and had to clean his entire hoard with a bowl of... spittle. Which was pretty stupid, seeing as he literally saved their lives from a damn army before that. But maybe, Typhoon knew about it, being the leader of the Black Dragons.
In Oathbreakers, Kethry happens to find a useless-looking dull-bladed old sword abandoned in a cabin in the mountains; she and Tarma speculate that it must have been a decorative sword and the gems and gilding were all stripped off by previous travelers, leaving behind what was left as junk. Kethry, on an impulse, takes it along when they leave, and it turns out to be the ancestral Sword that Sings, traditionally used to choose the proper ruler of the country of Rethwellan.
In L. Jagi Lamplighter's Prospero Lost, Miranda finds a chameleon cloak in a second-hand store. Both its presence and her discovery get explained later.
Atlas Shrugged has an anvilicious example, where Dagny Taggart and Hank Rearden discover a prototype perpetual motion machine abandoned and decayed in the gutted ruins of the Twentieth Century Motor Company. John Galt actually abandonedthe prototype, the plans and theoretical research that led to it because he designed it on the clock and they had a right to it - but more importantly, he knew that no-one capable of understanding it would ever work there again because civilization had ceased to value the original "perpetual motion machine" - the human mind.
Janet and Isaac Asimov's Norby series begins when young Jeff Wells buys Norby, a battered, apparently malfunctioning robot that the proprietor doesn't even think is worth selling. Said robot turns out to be a completely sentient alien artifact capable of antigravity, telepathy, FTL transportation, and time travel.
The time-travelling Glass in Septimus Heap is found in a warehouse where everything else is random junk like sheep bones.
Sherlock Holmes owns a Stradivarius that he bought from a pawn shop at a fraction of its actual value.
Myth-Gotten Gains starts out with Aahz finding a talking sword in a shop's discount bin. The sword claims to be part of a legendary set, and it takes some effort to convince Aahz of this. It takes even more convincing to get Aahz to buy it, as Aahz is a total cheapskate and doesn't need a sword anyway.
In The 10th Kingdom, a magic mirror that will allow the heroes to go back to their own world is for sale dirt cheap, as no one knows what it is. When its true nature is discovered, however?
This is more or less the premise of Warehouse 13 — it's essentially a Grail In The Garbage of the Week show. Your mileage may vary as to whether the eponymous warehouse itself qualifies as an "unlikely place". The Warehouse is more of a holding place. The Artifacts themselves are frequently found in odd places, like garage sales or the like. One Artifact is found in an old collection in someone's attic because they collected doorknobs and happened to find one from the Triangle Shirtwaist Factory fire.
As a show starring an antiques dealing rogue, Lovejoy often featured this without the supernatural aspect. In one example a church is facing financial ruin, none of the fittings are suitable to raise anything like the money required and as they resign themselves to failure the priest stops to feed his dogs. Cue the priceless antique he uses as a dog bowl.
Antique shows like the Antiques Roadshow and their international versions are the real-life versions of this trope.
In one episode of Antiques Roadshow, a man who was renovating a building that had, in the late 1700s, been a roadside inn into a house. In the attic he found a collection of old registers. Figuring they might be worth something to someone, he took them to be appraised without ever really checking them out. Turns out that the building had not only been an inn during most of the American revolution, but that the Founding Fathers of the United States apparently liked staying there and nearly all of them (including George Washington, John Adams, and Thomas Jefferson) had signed the registers multiple times. Later the man admitted that he thought they might be worth a couple of hundred apiece when he brought them in; the average appraisal put them at over one hundred thousand dollars apiece, and there were nearly thirty of them.
In another episode, a man had inherited an old home from an uncle and found an old saddlebag in the attic. When it was opened on the show, it had pristine examples of Native American tools and beadwork from the late 19th century, possibly received in trade. The presenter was halfway between drooling and shock.
In the Dutch show, Tussen Kunst en Kitsch, one expert even jokingly said to a person who bought a piece worth 3000€ for 2 BEF (0.05€) at a flea market: "I thought this was a fun show, this is just depressing."
Pawn Stars often have customers selling artifacts they found in their attic or at a yard sale, making the owner Rick Harrison wonder why he can never find something like that in a yard sale.
Cash in the Attic. Some of the stuff sells for less than expected, but sometimes truly rare and valuable items are found.
An orb that can restore people's souls is sold as a paperweight.
For that matter, Giles once bought a magical talisman from a sorcerer, convinced it was a knock-off. Not only is it real, it's used as part of an apocalyptic ritual.
A recurring sketch on That Mitchell and Webb Look involves a seller at a garage sale casually selling incredibly valuable artifacts for a low price. The Holy Grail itself, for instance, was sold for five pounds, as having already gained eternal life from drinking it, he sees no reason to keep it around. And later, the wardrobe that's the entrance to Narnia, as now that he has a garden at his new house, he doesn't really need the extra space.
Reality shows like Auction Hunters and Storage Wars have these found in abandoned storage units, when the owner couldn't keep up with the fees.
One episode of Stargate SG-1 features a random guy buying an Ancient communication device at a yard sale. Unlike a lot of these things, this is actually justified in-universe. Unless you have the right (fairly rare) genetics, it's functionally equivalent to a glass paperweight. It happens that the guy DOES have the right genetics, though...
In The Big Bang Theory, the geeks go to a yard sale and buy a box of grab bag items. One item turns out to be a missing prop from the Lord Of The Rings movie — The One Ring (well, one of three copies of the One Ring used in the film). Hilarity ensues as they all struggle to possess this 'precious'.
In one Malcolm in the MiddleCold Open, Reese accidentally breaks a cheap painting's frame. Before he glues the painting back down, he gets the chance to laugh at the name of the artist who painted the one framed beneath it: "Pic-ass-o".
An episode of Modern Marvels about garbage reclamation showed a box of seemingly ordinary gray dust; then we're told that it's over $1000 worth of platinum.
In Only Fools And Horses, the Trotter's become millionaires after the missing John Harrison Watch (which in real life, exists only as plans and may never have been built), ends up being found in their garage. Del likewise mentions he got it from an old woman who paid him to clear an attic.
One of the illustrations on the back of a Reader's Digest, entitled "Treasure Hunt", had a man at a garage sale looking at a bust of Lincoln. If you look closely, you can see a copy of Action Comics #1 sitting in a box of old newspapers.
Will inevitably happen sometimes in just about any game with a randomized loot system. Sooner or later, if not all the time, you're bound to find a magical weapon or artifact worth a fortune in the trash, unceremoniously stashed in a random barrel that looks no different from the last ten thousand you smashed, in a large pile of monster droppings....
Invoked constantly in The Darkness II, with Jackie collecting various incredibly powerful and amazingly rare religious artifacts just laying around New York. Sometimes justified by the fact that he is fighting an old cult that has dedicated themselves to controlling The Darkness, and would have collected these artifacts. Other times... why are the ashes of Cain hidden in a New York Cemetery? Why is the seashell containing God's lament at the great flood, lying on the subway floor? Why is a pair of swords forged from 2 of the angels of death in a mob warehouse? And why is the device for capturing and controlling the Angelus located inside The Darkness itself?
And WHY does Jackie's Aunt Sarah have THE BROTHERHOOD'S VERSION OF THE BIBLE?!
In Icewind Dale, the best longsword in the game, Pale Justice, is found on the corpse of a hapless adventurer in Dorn's Deep. What's more is that its inventory icon is the same as any regular longsword (by the time you're using + 3/+ 4 weapons) and shopkeepers will buy or sell it for a pittance.
Can be done deliberately in Evil Genius. You just stole the Ark of the Covenant? Eh, put it in the break room.
In Fable II, Murgo the merchant has the MacGuffin... he actually knows it's magic, but he has no idea what it can actually do. It is revealed in a DLC that it was given to him by your mentor, Theresa, which set the whole plot in motion.
In Dragon Age: Origins, Far Song, generally considered one of if not the best bow in the game, has apparently spent decades sitting buried in the stockroom of a tiny podunk blacksmith shop in Redcliffe. It's so well-buried that the only way to get it is to kill Owen, or get him to commit suicide by failing to find his daughter, and wait for his replacement to move in and go through his stuff.
Most Pokémon games have the Leftovers, one of the most useful held items in the series, hidden in a trash can.
Somewhat justified in-universe. It is just a small pile of half-eaten food...that just so happens to be able to be able to regenerate itself, giving the Pokémon an infinite supply of free health.
In Borderlands, you can sometimes find a really good gun when you open up a garbage can.
Team Fortress 2 has a heavily hat-based in-game economy between the players, where some virtual hats can cost several hundreds of real life dollars. This also means that there are varying degrees of currency, including using other expensive hats as a large denomination of metal (the primary currency) to free up spaces in one's backpack. One of the common things newbies do is to trade away a pair of earbuds or a Bill's Hat for a bunch of weapons. Both are promotional items that seem worthless, but on the hat market are worth 40 and 20 real life money (or 40 and 20 in-game refined metal). By comparison, any given weapon in the game is worth 1/18th of a buck (18 weapons to make 1 refined metal).
Happens often in The Legend of Zelda: Skyward Sword, not only because the involved items do seem unimportant, but also because Link is actually unable to carry them on his own. And very fittingly, the player doesn't happen to suspect about these items until Fi's dowsing ability indicates that they are indeed important:
In the sacred spring behind Skyview Temple, there are several waterfalls adorning the place. One of them is spewing the Sacred Water Faron needs to heal her wounds.
During the search for the Key fragments through Eldin Volcano to open the Earth Temple, Link gets past a seemingly out-of-place metallic pinwheel in a crag that houses a watchtower. This pinwheel is necessary to make one of the wrecked windmills work again in Skyloft, so a podium activates and Link can play the Song of the Goddess there to gain access to the Thunderhead.
Also in Eldin Volcano, there is a crystal ball that adorns the entrance to the Earth Temple. A good replacement for Sparrot's crystal ball when it breaks.
In Lanayru Desert, there is a color wheel in a hill southeast. This is the wheel Dodoh lost while he was preparing the last details for his flight minigame.
Character-based example: The seemingly mute dinosaur in Star Fox Adventures, located next to the underground caverns south of Thorntail Hollow is the fourth Gatekeeper, giving access to Dragon Rock.
In Dead Island, the vendors will sometimes sell white, common weapons, which have much, much better stats than the rarer, colored weapons. For instance, you could see them selling a blue colored machete which does 500 damage, then the white colored machete right below it does 650 damage...
In Drowtales, Ariel ends up "purchasing" a random slave-girl. Said girl is posessed by the QUEEN; she was dethroned and used the slave Ragini as a living phylactery. Also, Ariel's friend Faen has some to-be-seen superpowers, too bad they killed someone by accident.
In Petty's spinoff of Nuzlocke Comics, Barb the Nidoqueen collects pieces of paper she finds. At one point, trainer Locke is depressed over recent deaths, and Barb tries to cheer her up by sharing her collection... in which Locke finds the SS Ticket and the Bike Voucher. To most Pokémon players and to Locke, these are priceless, but to Barb, they're just paper.
The Angry Video Game Nerd acquired a copy of the gold cover Nintendo World Championship, the absolutely rarest game in the world in a sale where the previous owner bundled it in with other other much more common games. The bundle contained two copies of the game, the other being a much more common reproduction.
Some of the artifacts held by the SCP Foundation were discovered in this manner. In fact, certain documents suggest that the Foundation's "standard channels" for discovering potential SCP objects take this as a rule rather than an exception, especially for mundane-looking objects with more subtle properties.
A frequent result of Gaia Online's economy system. Limited-release items such as Monthly Collectibles or subsidized advertisement freebies can become popular and explode in value long after most users who actually got them have stashed and forgotten them, or possibly even left the site.
In one episode of The Real Ghostbusters, Ray finds himself in possession of the shears belonging to the Three Fates, finding them on the ground just as he needs to cut something. He keeps them, forcing Clotho to chase him all over New York to try to (discreetly) get them back, because she's the one who dropped them in the first place. As far as he can tell, they're just a pair of scissors, but they're really awesome scissors.
American Dad!: In "Return of the Bling", Roger finds what is apparently the One Ring near the site of a plane crash... and then promptly throws it away.
Roger: It turns you invisible in the middle of nowhere? What good is that? Where were you when I farted at Danny's wedding?
There's another episode where Moe sells Marge a dusty bottle of wine he has lying under his counter, only to find out that its one of the last of its kind, and insanely valuable. He then proceeds to wipe his tears on the last copy of the lost Shakespeare play.
In another episode, Martin's mother tries selling some random stuff she found in his room to Comic Book Guy. Said "stuff" being the original handwritten script for Star Wars, "Princess Leia's anti-jiggle breast tape" and a film reel labeled "Alternate ending — Luke's father is Chewbacca". CBG tries to scam her by offering 5 dollars only for Bart and Milhouse to inform her that it's worth thousands. Mrs. Prince replies "Well! If this is valuable, then back to the leaky basement it goes!"
Earthworm Jim does something similar to the above, with a literal Holy Grail included in the treasures.
In The Venture Bros., Dr. Venture and Billy Quizboy find the ORB, an artifact which might have devastating consequences just sitting out in the open. It turns out it was broken for about a hundred years and completely useless.
Adventure Time did the same int+ thing with the nerd glasses. The duck knew that they did make people smarter, but sold it for a JOKE. This is justified in it would be considered a cursed item, as Finn goes crazy after wearing it, but temporarily learns enough about universe-atom theory to make P.B. FALL IN LOVE WITH HIM.
The Planet Express gang from Futurama witnesses an auction house sell the universe to "the being of unimaginable horror." He's the only one who puts in a bid.
It happens in real life more than you think. More often than not, the people who sell at yard sales or flea markets generally don't have that much of an idea on how valuable the junk they're selling is. Now, thanks to shows that play up this trope, people are getting a little bit wiser; however, sometimes they just put large price-tags on junk and hope for the best.
The Passion of Joan of Arc, one of cinema's greatest classics, was thought to have been lost until a nearly perfect print turned up in the closet of an insane asylum.
Oskar Schindler's list of names (the real one) was found in an attic in the late 1980's.
The winning $200,000 game-piece for a 1995 Wendy's contest was found on a discarded fries container by garbage man Craig Randall.
A Tennessee man visiting a museum gift shop wound up buying a copy of the Declaration of Independence as a souvenir, but after noticing it didn't seem all too fresh, he had it appraised. It turned out to be an original draft, one of the few in existence, worth almost as much as the genuine document.
One of the lost episodes of early Doctor Who was found in a church basement.
Fishermen in South Africa were routinely catching a certain ugly, inedible, useless fish, which they threw back (unless they needed sandpaper — its scales did quite nicely). Then a marine biologist recognized the ugly, useless fish as a coelacanth, of tremendous value to the scientific community.
The Stanley Cup has turned up, on separate misadventures, sitting on a frozen canal overnight, used as a flowerpot by a photographer's mother after accidentally being abandoned at a portrait studio, and left on the side of the road after the winning team had to change a flat tire.
Sometimes even the garbage itself is valuable to the right buyer. Scrap metal dealers will pay handsome prices for broken-down junk that's made of metal they can re-sell. Bottle depots will buy aluminum cans and glass bottles for the same reason. Ambergris is essentially a sort of stomach acid produced by whales that stinks to high heaven, but was once highly valuable as an ingredient in perfumes. Manure and guano, two different types of animal poop, have been used for centuries as natural fertilizer.
A Sturmgewehr StG 44, the grand-daddy of the modern assault rifle, was encountered at a Hartford Connecticut police gun buy-back. The police who accepted it knew what it was, and convinced the owner to hold on to it and explore options of selling it to a historical museum instead of subjecting it to the melt-down that all the other guns at the buy-back were heading for.
In May of 2013, an old scroll that had been lying around in plain sight in the Bologna University library was identified as an 800-year old copy of the Torah, likely the oldest complete copy in existence, and possibly worth millions.