Let me explain more clearly (i) they quickly hoard together good books at the sale, cutting off access to other prospective buyers on location, like me, (ii) they buy a very small percentage of the books they hoard (guy in the OP says 1 in 30), the others books are just left in an unordered pile, so it's hard for other people to find them, and finally (iii) they create a mess for the sale organizers, I've heard many organizers complain about them and some sales won't let them in.

It's like a guy who runs in a cafeteria, gets all the good desserts and then tries to sell them to you.

Well sounds like the people organizing the book sale are doing a bad job keeping their patrons in check, then. No matter where you go, there are always going to be assholes. I have friend & family who own bars & restaurants - I can tell you that a guy trying to monopolize the dessert buffet and causing annoyance to the other guests is going to find himself outside on the pavement real quick.