This is one of the many reasons I do not miss working for a bookstore. A large percentage (but not the majority) of our customers were either grifters or thieves.
They sell them to other bookstores. Or try to, anyhow, and that is not necessarily a good idea, since (especially in the used and rare bookstore world) owners often know each other. I worked at a used bookstore and a friend of the owner who had their own store was robbed: a few days later, the thieves showed up with boxes of books taken from that store. Thankfully the owner of my store realized it right away and the thief was caught.