Imagine that if JimmyTheGoon (an admin) wants, he can drop a chest, fill it with what he wants to sell in the top row, and what he wants for each thing under it (two rows), and tack a sign to it that says "QUICKSHOP" on the first line, and "JimmyGear", or whatever he likes, on the next. Now, anyone can trade with the chest Jimmy put down via the command, say: /quickshop JimmyGear. And the villager GUI pops open, with his trades. They could also add the line "INFINITE" to the sign, meaning the trades keep on forever. Or maybe that'd be the default, and "FINITE" for "it runs out". Like that.