Karl Marx Hates Your Guts is the inverse of Adam Smith Hates Your Guts. Goods are available and distributed at the same monetary rate everywhere. "Buy Low, Sell High" doesn't apply in such a place. Prices are fixed in such a way that it is impossible to make money by buying something and then re-selling it elsewhere.
This becomes a problem in these situations:
The only way to earn money is viaMoney Spider. If speculation is unprofitable, upgrading equipment becomes dangerous.
This applies only to games or stories where you could make a living as a businessman if the game did not set up prices in precisely the right way as to make this impossible. The trope is named after Karl Marx, one of the founders of modern communism.
This can also, ironically, overlap with Adam Smith Hates Your Guts when there is both a single price no matter where you go and that price rises as you move along (or a variation on that scheme).
This is somewhat Truth in Television as the act of arbitrage will cause prices in various areas to move to one price (the so-called law of one price). However, the way this is handled in games makes this somewhat infuriating, particularly when prices should be different despite arbitrage (or because arbitrage can't happen). For instance, having prices for a night at the inn fixed across the empire makes no sense if one inn is in a major Hub City and another is in some village nobody has ever heard of.
In The Legend Of Zelda Twilight Princess, the shops have varying selections, but everything that is sold has the same price regardless of where you buy it... with three exceptions: the Gorons who set up shop in Castle Town, who sell Red Potion, Lantern Oil, and Arrows at a 10-Rupee markup from the standard (they call it "regional pricing"); the other shop before it becomes a branch of Malo Mart, which sells a good selection of things, but at a higher price than even the last wallet upgrade can hold; and Malo Market Castle Town, which has the same selection at half the standard price. Of course, you drop some serious Rupees making these discounts available...
In World of Warcraft, vendors pay 25% of the selling price of an item, if you're allowed to vendor it at all, and all vendor-bought items cost the same everywhere (subject to reputation discounts). Sources of income are quests and Money Spiders. That said, the player-driven economy is vibrant and can be highly profitable.
The majority of MMORPGs will have NPCs pay the same price for the same item regardless of location. In the few games that vary store prices with location or a review of the current (in-game) economic situation, arbitrage becomes a common if rather inefficient source of income for players — though merchant classes can usually squeeze more out of it due to trade being their main activity and prices don't fluctuate as much as in Real Life if they change at all.
Kingdom of Loathing has a "mall" where players can sell items to other players. This trope often ends up being invoked, because all items have a minimum sale price (100 Meat or twice the NPC-sell price, whichever is higher), and for most items, the ideal price (where demand matches supply) is much less than this. Selling these forcibly overpriced items in the mall is impossible, unless you spend ridiculous amounts of money on advertising so that your store appears above the thousands of other stores selling the same thing at the same price. Even then, there's usually no way to sell such items for more than you paid for them elsewhere. (Through the mall, at least. The best way to sell such items is by looking up who has the most of them in their public Display Case, and individually ask them whether they want to buy your stock directly, without involving the Mall.)
While the economy for player fortresses in Dwarf Fortress is disabled and pending a significant retooling, with foreign merchants (the ones you visit in Adventure Mode) all prices are fixed no matter where you are, based entirely on material and quality, and every non-worthless item has a minimum value of 1. So while you can't turn a profit on bought goods, even if they're from the other end of the world, you can make money selling found/stolen goods even if they're literally found in infinite quantities laying around on the ground in front of the buyer. The newly begun Caravan development arc is largely based on averting this by implementing a value system based on the availability of goods in the area involved.
Completely averted in the latest Atelier series game: Atelier Lina. The price of goods fluctuates year round, and buying stuff in one town to sell in another town is the fastest, easiest and most profitable way to make obscene amounts of cash in a very short amount of time.
Averted in Paper Mario, where it's possible to buy items in one town and sell them at another town for a profit. One of the possible ways to do this is told to you as payment for a side quest. "Go to Petalburg, buy a Sleepy Sheep, go to Rogueport, and sell the Sleepy Sheep for a two-coin profit!"
Using logic and the principle of supply and demand and how it affects price, this can be cranked Up to Eleven. In Keelhaul Key, buy Fire Flowers. Sell them in Fahr Outpost for a 3-coin profit. Before leaving, buy Ice Storms, which sell for a 4-coin profit in Keelhaul Key. It's easy to make money in the late game without killing enemies for it.
Partly averted in all Suikoden games since II with regard to trade goods, where you can buy low in one town and sell high in another. Played straight with most items, of course.
One town in Dragon Quest IV is short on armor for a while, and will buy armor for far more than it costs, at least until each type of armor is sold enough.
The merchant Taloon is also able to invert this for a time during his chapter after setting up his own shop: his wife will sell any goods you give her at a much higher price than any of the other shops around. Lady must make one hell of a sales pitch... Needless to say, this can be abused to Game Breaker status, if you know how to work the system...
Earlier in the same chapter, Taloon is a salesman at a weapons shop, and can haggle with customers to get more than the usual price. Or to prevent them from buying an item that you want to buy yourself.
Also averted somewhat in that the Casino charges different prices for gaming tokens depending on what chapter you are in. The cash-strapped Tomboy Princess gets discounted tokens for only 10 gold each, whereas the aforementioned merchant has to pay 200 gold each. In the final chapter, the Hero's party pays 20 gold each. This is important because regular cash doesn't carry over from chapter to chapter, but casino tokens do. Thus, it's in your best interest to convert as much money as possible into tokens (which can be used to purchase some of the best equipment in the game if you accumulate enough of them) before completing the chapter. Since a patient player can accumulate an endless supply of gold via Taloon's shop, it would be something of a Game Breaker if tokens were being sold at the normal price.
Played straight in Xenosaga, in which all goods cost the same amount of money everywhere, and no goods ever increase in price. Acknowledged and possibly averted somewhat be a subplot that involves possible cuts in price depending on free-market investing on Shion's part. However, a shop is a shop is a shop in Xenosaga.
Which makes perfect sense, as their version of the internet, the UMN, includes a FTL teleportation system that can transmit non-living objects. Where you are really shouldn't budge the price, you can download it anywhere!
Inverted in the Tales Series, going all the way back to Tales of Phantasia. You can break the game's economy horribly by buying and selling stuff in bulk in the right towns.
The Elder Scrolls series is a rather egregious offender considering one of the big selling points is being a sandbox that allows for more playstyles than endless dungeon-crawling. But you're pretty much limited to alchemy and thievery, the latter of which isn't as lucrative as it sounds because nobody has anything of value in their houses. While it would make sense that items would be cheap in the big cities' trade districts and more expensive in little podunk shops with supply problems, prices are set by item so they remain basically the same, excepting some skill-based variation and how much the merchant likes you, no matter who's selling.
At least in Morrowind, if you have high mercantile skill and the merchant likes you, you can buy something and sell it back to the same vendor for profit.
Dragon Age: Origins — sale and purchase prices are identical wherever you go in the world (even though availability varies), and the only crafting item that makes a profit (i.e. the finished product can sell for more than the components cost) are high-tier Lyrium potions. One merchant in the entire game averts this, and only if you're a Dwarf Noble.
Averted in Secret of Evermore: In the same bazaar where you get the trading quest chain, there are loops that allow you to gain money (by buying cheap goods, trading those goods for more expensive goods and then selling the more expensive stuff to an appraiser).
Averted in Fable, where you can make money through arbitrage, or (thanks to a Good Bad Bug) sometimes even buy things in bulk and sell them back to the same shopkeeper at a profit.
Particularly bad in Geneforge: While the price to buy a good may vary from town to town, the price you get selling that good is the same everywhere, and it's less than the cheapest amount you'll pay to get that good.
Averted in Fallout and Fallout 2. The price you sold things for was fixed (RadAway always sell for 500) but the buying price varied depending on your Barter skill, reputation, and the general disposition of the merchant in general. In several cases you could actually selling things for less than you bought them from the same vendor, allowing you to literally clean out the store by buying everything and selling back smaller and smaller shares until the merchant was left with a single chip or a worthless junk item.
Fallout 3 and Fallout New Vegas have variable buying and selling prices (as your Barter skill improves, both of them converge on the true "value" of the item). However, location and relative rarity don't factor into it; Little Lamplight might be the only place you can find radiation-absorbing superfood Cave Fungus, but the traders in the next town over will pay just as much as the ones all the way across the map. Certain communities will offer rewards for specific resources they're short on (Little Lamplight needs "Strange Meat" to grow the aforementioned Cave Fungus) but only if you give them to the right person — the town merchant will offer the same price as they would anywhere else.
Followed fairly straight with the regular merchants, but averted with the robot traders on Sender Station. (They're not trading in robots, they're actual robots who trade stuff.) They only trade various useless luxury items (and also lifepetals, but you need them for other things) but each have different price listings. Simply buy as much as possible of a ware where it's cheap, then hoof it over to the one who buys it for a high price. Then repeat, but now you can buy even more. If you can stand the repetitive clicking, you can make your party economically independent in less than thirty minutes. And you can always go back if you need more.
Averted in Saga Frontier for Gold Ingots, which will rise in price the more you buy and drop in price the more you sell; in fact, you can abuse it to make yourself obscenely rich.
Merchants in Albion may have varying prices. Some even have two separate inventories for selling and for buying that have different exchange rates, but the rule of thumb is that regardless of the exchange rate, they will give 20% less for everything you sell them, then what they would ask when you're buying the same item. The way it's set up, buying something from the cheapest merchant then reselling it to the most expensive merchant will most likely get you the price back, but not much more. On the other hand, there are a LOT of merchants who are more than willing to buy your hard earned loot and Vendor Trash, for pocket money (read, half the price an average merchant would give you).
In Final Fantasy games, items can generally be sold for half the purchase price. However, in Final Fantasy VIII, the player can learn the "Buy Low" and "Sell High" abilities of the Tonberry GF, which allow the player to buy and sell items at 3/4 the standard price. With those abilities and the Carbuncle GF's "Recov Med-RF" (Recovery Medicine Refinement), you can buy Tents and Cottages, turn them into Mega-Potions, and sell those for about a 20% profit.
Final Fantasy IX has the so-called Cotton Robe trick, wherein you can buy Wrists (for 130 gil), Steepled Hats (for 260 gil), synthesise them into Cotton Robes (for 1000 gil), and sell those (for half the 4000 gil purchase price, 2000 gil). Result: 610 gil profit. By two more iterations you'll have enough profit to process Cotton Robes two at a time for 1220 profit. Twenty goes can turn an initial 1390 gil into over 1.3 million.
Final Fantasy X-2 has a Game Breaker exploit that happens early in the game: if you rescue merchant O'aka from his creditors and then buy enough from him to pay his debts, he rewards you by selling things to you at a discount... which puts every object at a lower price than what the bartender right in front of O'aka buys from you. Buy 99 of everything from O'aka, sell to Barkeep, and see the money rake in!
In Guadia Quest of Retro Game Challenge, there are bars of precious metals that sell for the same price they cost. However, these are useful, as they prevent you from losing money if you are wiped out.
Full on averted in Sid Meier's Pirates!. The prices of each good vary from port to port depending in unchanging factors such as what they produce and what they need, which usually match up with the actual history, and variables like the wealth and size of the town. It's possible to play a straight up trader, or to attack shipping routes to drive the wealth of a city down, or even play protectorate and nurture a few cities into wealth.
And it's not just wealthy towns that are good business. While a rich town may want sugar and spices, poorer towns will want Goods and other such commodities. Then again you could just attack ships and duel people.
Few things are more disheartening in the game than returning from a long voyage to discover than pirates have pillaged the town and rendered your cargo of sugar near worthless.
Averted in Mother 1: eventually, you encounter a man who will fill up your inventory with mouthwash, and his assistant will sell you more for only $10 apiece. If bought from anywhere else, Mouthwash costs more than fifteen times that amount, yet the mouthwash you get from the man can be sold for the normal price of half what it's worth at the store, thus allowing you to make a pretty good profit.
Aerobiz: Though you can sell old aircraft to "World Lease" at half-price, you can never purchase any used aircraft to bolster your fleet, and all aircraft are sold at a fixed price which never moves even as the design ages.
In Dungeons & Dragons, everyone uses the same currency and goods tend to cost the same everywhere. Pretty much an Acceptable Break from Reality given how much trouble having to exchange bits of your vast fortune everytime you left the country would be. Not to mention that sheer amount of head-scratching that goes into a spell that costs "20,000 gold pieces worth of diamonds" when the amount of actual diamond that is varies wildly depending on how you calculate it.
There still ends up being little holes in the game's default price lists that allow for capitalist enterprise (assuming the DM is a pushover.) For example, a ten-foot pole costs four times as much as a ten-foot ladder, so even if you can only sell items at half price, you can turn a profit by buying ladders, breaking them up into two poles each, and selling them.
Averting Cut Lex Luthor a Check with spells, a character could fill a warehouse with trade goods with Fabricate. Just hire near free unskilled laborers to collect the raw materials, spend a minute casting the spell, and then have a couple of skilled workers to handle the books and some guards. You'll make thousands. If you also know how to make masterwork weapons or alchemy, dropping masses of masterwork longbows or alchemical goods on the city could be even more hilarious. Offer to equip the king's archers for half cost. Since Karl Marx Hates Your Guts and you can only sell them at half price, you should sell them in no time flat. The spell costs 1/3 the value of the goods to create them and you are forced to sell for half, but remember - you can make 90 cubic feet of goods in a single casting at the minimum level. Or 9 cubic feet if you make minerals. That's about 4,400 pounds of steel goods. That's almost 9,000 masterwork daggers, if you are good enough to make them. Dozens of spells are ripe to be abused through these kinds of mechanisms and are not so absurd as cornering the ten foot pole market.
In 4th edition this gets even worse: you are not allowed to sell an item for more than 20% of its buying price. This is because enchanting magic items is much easier and cheaper in 4e than previous editions, but you're supposed to be exploring dungeons, not mass-producing +2 longswords.
This particular quirk might be explained if the adventurers always purchase magical items "on commission" - if there is low market liquidity, you might have to lower prices a great deal in order to move surplus stock quickly, and most items purchased will be made-to-order rather than off-the-shelf. (This may be giving too much credit to the game designers.)
The official handwave for the low selling price is that trafficking in magic items is very risky and expensive — it's hard to find buyers since adventurers are rare, and magic items are high-profile targets for bandits and thieves.
This leads directly to one of the worst aspects of the 4th edition: The official adventure paths are VERY stingy with giving out magic items to the players, only one in every couple of encounters. Every player needs at least three magic items (Weapon, Armor, Amulet) to be up to date to be competitive, so it hurts a lot more than in previous editions if you happen to find an item you cannot use. Like, finding a +3 Longbow when no one in your group is an archer. If you try to sell or disenchant the item you only get 20% of its value, which makes this highly ineffective, and you lose a valuable item without getting a proper compensation.
Magic items are intended to be rare and you aren't supposed to be required to have them at all. During third edition, a vicious circle developed between magic items being too easy and enemies being balanced to acknowledge how easy magic items were to get, which led to magic items being even more common for defeating those enemies causing even more powerful enemies with even more powerful magic items to emerge.
It also makes rust monsters, formerly one of the most frightening creatures to a fiscally conscious adventurer, into an extremely useful pet. In 4e, magic items can be disenchanted by a ritual that yields a powder containing about 20% of the magic essences needed to make the item. However, if a metallic magic item is fed to a rust monster, it leaves all the magic behind in powdered form.
Similarly semi-averted in the Escape Velocity series. While ship upgrades cost the same wherever they are available, trade goods are available for different prices on different planets, and random events can drive the prices further up or down. Certain routes are known to be such good money generators that cargo space itself will quickly become the limiting factor in how rapidly you can accumulate wealth (with a 50% margin between buying at "lower" and selling at "higher" it becomes a matter of finding an expensive enough good to fill your hold with for it to even be worth your time later on).
Played straight, however, if you try intra-system trade in Elite 2 or FFE. The prices on all planets of the same system are the same, unless some commodity is in dire need somewhere (but then again, most likely this commodity is unavailable on any planets of this system). The only ways you can make money without FTL travel are mining and waste disposal, and both have very low profits.
Averted in Sub Culture, where some items are bought higher in some places than they are sold in others. Refined thorium notably is sold cheap by the refinery and can be resold for a good price in cities.