You enter a familiar town, welcoming a break from the hectic adventurer lifestyle. Though you discover a shop you haven't seen before. The Copper Merchant.
Looking inside, the shop carries a massive variety of wares. Ranging from what could be useful adventuring supplies, to children's toys, to miscellaneous trinkets and knick-knacks. All of it incredibly poorly crafted and low-quality. Speaking to the merchant running the store, you'd learn that they ask for only a single copper piece for each object. You get what you pay for. A smart adventurer would leave, everything clearly of no use.
But you didn't. You bought something. Not something you could even attempt to make use of. A toy sword, for children. The dull, oddly-shaped "blade" was made from a strange transparent resin, the hand guard had the shape of a pig's head molded into it, and the hilt had a music box somewhere inside activated by a button on the side. Its sheer existence was simply so bizarre, you had to have it. You gave the shopkeep a copper coin, and left the shop strangely satisfied with your absurd, useless prize.
This object was suppsedly worthless, though you had seen worth in it. Worth brought by the very things that another would deem makes it worthless. But as you eventually left to continue your journey, this worth you saw manifested into a magical enchantment.
On its own, it wouldn't do much better than a small club in combat. But upon pressing the button, the "blade" lights up into a variety of shifting colors, giving off light for a good distance. Attacking with it would harm your opponents with pure radiant energy, in addition to the harm that comes with being hit with a chunk of what you're unsure is really resin. Even the music from the hilt seemed so much more beautiful.
Truly, this weapon was a gift straight from the gods.
The Copper Merchant (name pending) is a fantasy dollar store. It sells cheap crap1 for one copper coin (a dollar) each. Everything there is, from a pragmatic standpoint, worthless garbage. But some of this garbage is magical. Not all of it, but some.
Determining which wares are and are not magical is incredibly difficult. Even a grand wizard, trained for decades in discovering and identifying magical artifacts, would find the task nearly impossible. The reason for this is simple: If you're looking for something with great magical power, you're inherently looking for something valuable. But every single item for sale is, by nature, of little to no value whatsoever. A high wizard would not concern themselves with children's toys and poorly-made clothing. But if you purchase this garbage because it is garbage, because of the poorly translated slogans on the shirts, because of the poorly painted misaligned face on the dolls, because of the toy swords bearing the likeness of great heroes2. You see value in it. Even if you are to steal such an item, you are clearly demonstrating that you see some sort of worth in it. A desire to have it.
This is what enables the magical abilities in the wares that have them. Not while they are inside the store, but once you have left with them. When you know for sure that you are going to keep this bizarre object you kept. Your hilariously mistranslated shirt granting you protection as if it were armor, without the weight or otherwise impaired movement that would come with it. The doll with the misaligned, poorly painted face animating as a minature golem. The swords granting you a small amount of extra power in combat, in ways completely unrelated to the heroes who's visage they bear. The shop probably disappears or something when you come back for more. That's just the standard for magical shops.
Implemented into a tabletop game, what this would be is an excuse to give your players magic items. The amount of magic items given being determined almost entirely by you. The players can decide how much garbage they buy, but only you can decide how much of the garbage is magical. Like a club that deals extra radiant damage, gives off light for half the distance of a torch, and plays the Crazy Frog remix of Axel F. Or a tunic that grants you extra armor. Stuff like that. Plus, the players can't put you on the spot to decide what's magic before they buy anything, because they'll fail to find anything if they search for value.
This is kind of a stupid concept but I like it.