The Wandering Shops or "tabernae vagantes" are exactly the kind you get in those "Lies-To-Children", called Fairy Stories.
Certain constants apply to these shops. They carry exactly what the person is looking for, even if they aren't looking for anything particular; Out of all the myriad items, most of which are broken, the most powerful and mystical one will be the most ordinary looking, and the most affordable; and the shop owner will be older than mud, but sharper than flint.
These shops will appear in rarely travelled side streets, and will look as though they have been there forever. The next time the customers return (often to return the purchased items) the shop will be gone, the space it occupied looking as though it has been that way forever as well.
Quantum is suspected to play a large part in this, though some people believe it's just a clever way to avoid various Sunday Closing laws, while others believe these shops form an Emporium, a galactic empire controlled by shopkeepers.
Tempting thought these explanations are, and while noting that they seem to fit the observed facts, they are almost entirely wrong.
The classic example of the wandering shop is Skillet, Wang, Yrxle!yt, Bunglestiff, Cwmlad and Patel, whose premises, in The Light Fantastic, are used as a plot device to ferry Rincewind, Twoflower and Bethan back to Ankh-Morpork. A similar establishment appeared in The Backs, by Unseen University in order to connect Imp y Celyn with a particular guitar.