Terry Pratchett. Small free folk (№ 333549)

They looked like a traveling Tinker, but she knew none of them would be able to solder the pot. They earn a living by selling an invisible product. And after the sale of this product still remains in them. They sell things people need, but often people don't want to. These strangers offer to buy the keys to the Universe who never heard of the existence of the Universe castle.