During his trip to the town of Ar-Piji, our hero stumbles upon a mysterious shop. It contains all sorts of oddities, but for some reason he's the only one in there except for the hagged old shopkeeper who keeps cackling and petting her black cat. As our hero examines an item, she calls out, "Ohh, you'll like that one. That's [convenient plot artifact X]. Been around here forever. I'm in a generous mood today, so for 2 coppers it's all yours." So our hero purchases the item. When he returns later, possibly to inquire about the item's origins, the shop has disappeared *le gasp* and is promptly forgotten about for the rest of story.
I want a world where such artifact shops are commonplace (disappearing optional). Adventurers can find all manner of items in one ("By the gods! A Broadsword of the Archangel's Fury +7!") which may or may not be plot-centric. Some items are truly unique while others are common items with a rare combination of properties or enchantments (affixes, in RPG terms) or are simply rare items themselves. There is also some sense of buyer urgency behind the idea that items are rare or one-of-a-kind and even more urgency if the shop is only present at a location for one day.
Part of the problem is supply. The whole idea behind the disappearing item shop is that it's a plot device to place an item in the hero's hands without explaining its origin and removing all ways of finding out its origin. With a multitude of these shops, it becomes harder to handwave the supplier away, at least from a worldbuilding perspective. There would need to be a way to have a consistent supply while more or less maintaining the "unique" aspect of the items.
The other problem is maintaining the fantastic appearance of these shops. In our world, we don't go into a pawn shop and find it filled with dazzling goods. We might find a few nifty knick-knacks, but the majority of the goods are mundane even if they are unique. So how can I make these shops alluring, yet common?
Is it economically sound to have shops filled with unique artifacts throughout the realm?
Where would these shops get unique artifacts?
Is it possible to have these shops be common, yet each one is still unique and exotic?