Why will only select individuals purchase treasure-type items despite so many people selling them? Well duh, you run a shop in an RPG. You're the endpoint for useless knickknacks that exist only to generate gold for the adventurers that find them, be grateful anyone buys them.
Why are you only able to take a small number of things back with you if you fail at the dungeon? There's no way Recette could drag an unconscious person in full armor by her frail little self and the teleportation spell probably has a weight limit close to the combined weight of Recette and her companion.
It's explained in-game that the merchant's guild supplies a magic little box that can hold an item, but still be carried out. Presumably, more merchant experience allows Recette to cram more stuff in there.
Why does the Guildmaster, your supplier, keep coming to you for various orders and goods that he could very well easily get for himself? When it comes right down to it, Recette is still a little girl trying not to starve, and the Guildmaster knows it. He's subtly trying to help you out of the gutter by buying from you.
He's probably the Guildmaster because he likes trading; he's probably coming to your store for the thrill of haggling. This would also explain his rather dour expression when Recette gives him a price that he accepts without haggling - he didn't really have fun with it.
Certain customers will only show up if you have the right "atmosphere" in the store.
The little girls and old men share like "Light" and "Plain" atmospheres due to their simplicity and somewhat low budgets.
Euria (a scammer) will only appear where she thinks she can make a lot of money; Gaudy stores.
Griff (a demon) and Caillou (a mage interested in heretical artifacts) will only show up if you have a "Dark" setting.
Louie, being a bubbly and a perpetually happyIdiot Hero, prefers light stores, and so does Elan, who is a priest. Both also have little money, so they prefer "Plain" cheap-looking stores.
Charme enjoys gaudy stores, but being also a thief she also is much more likely to appear in less-Light settings.
There's a bit of stealth roleplay involved in the game: Tear actively encourages you in tutorials to gouge prices and haggle hard, but keep in mind that she's a fairy accountant with her mind on the bottom line and a (barely-concealed) sense of greed. The game rewards you more for roleplaying like Recette: treating the customer right, giving them fair deals and building friendly relations with them.
Alouette is the daughter of a major chain store and could probably acquire any item that she wants through that. So why does she frequent Recette's store and buy expensive items at ridiculous mark-ups? Because she secretly wants to befriend Recette, and the best way to give her money is to buy the most profitable items in the store.
Well you get Jelly from killing the giant jellyfish boss. Maybe he`s a badass boss runner too.
Or he gets his supply of jelly from adventurers. Don't forget — this is an RPG world, which means that people are giving out sidequests.
Recettear shares a world with Chantelise, and there, a single power crystal can be used to cast a fairly potent spell. Tear confirms the experience gems in this game are the same objects. The amount each adventurer absorbs for a single level, they would be an unstoppable Juggernaut by Chante's standards.
Obviously, it's because none of the ADVENTURERS are using a Fairy Companion to cast spells!
It was suggested that foodstuffs never spoil in accordance with typical JRPG mechanics, which explains why customers are able to sell them. That explanation still does not cover the foodstuffs that were apparently "used with love" before being sold, though.
You may get a case of this if you start wondering why there are no apples at any step in the "apple pie" fusion. Thought it's not that different from Real Life blueberry pies.
Recette was alone with no way of supporting herself for three months when her father left. She has, as Tear notes, a very thorough knowledge of what plants in the town square are edible (telling Louie which ones can harm him, even). She constantly talks about her appetite. It's very likely that Recette was close to starving before Tear showed up.
She also has, after a conversion of the house to a store that probably came out of Tear's pocket, 1000 pix. That'll buy you ten of the cheapest food item in the game, assuming you're using a merchant's guild membership to buy in bulk from the source (at the prices normal people pay in the store, more like 8). And considering that dear old dad was at least five weeks away from returning home...