'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, the shop will be gone, the space it occupied looking as though it has been that way forever as well.'

 

T. Pratchett