It reminds me of another used bookstore experience I had... In the town I previously lived in, just before I moved.. I gathered together a box of books I didn't want to deal with, and decided to take them to the bookstore in hopes of getting some cash for them. Out of the entire heaping box, they wanted like 8 of them. And, in my opinion, not even the best of the bunch. Anyway, they would give me either $1.50 cash or $5 store credit for the ones they wanted. I found one book - a like-new copy of my favorite book ever - that I wanted, and called it even. They offered to dispose of the rest of the books for me, but wouldn't pay any more for them. So I ended up going back home and throwing them into the dumpster. Which really hurt, because I respect books so much. But I'll be damned if those cheap bastards are going to make money off of my books without giving me anything for them.
In any case... That's why I tend to keep the books I buy. I usually buy very carefully, only getting certain authors or series books that I'm sure I will like. I don't mind only having to pay half price to get them, but selling them back is just a big ripoff.