There have been three general theories put forward to explain the phenomenon of the wandering shops or, as they are generically known, tabernae vagantes.
The first postulates that many thousands of years ago there evolved somewhere in the multiverse a race whose single talent was to buy cheap and sell dear. Soon they controlled a vast galactic empire or, as they put it, Emporium, and the more advanced members of the species found a way to equip their very shops with unique propulsion units that could break the dark walls of space itself and open up vast new markets. And long after the worlds of the Emporium perished in the heat death of their particular universe, after one last defiant fire sale, the wandering starshops still ply their trade, eating their way through the pages of spacetime like a worm through a three-volume novel.
The second is that they are the creation of a sympathetic Fate, charged with the role of supplying exactly the right thing at the right time.
The third is that they are simply a very clever way of getting around the various Sunday Closing acts.
All these theories, diverse as they are, have two things in common. They explain the observed facts, and they are completely and utterly wrong.