At California State University Long Beach, I usually got a pretty good deal from my textbook buybacks. Though, I remember one year I had to buy a brand new (used wasn't an option) $90+ anatomy textbook. When I took it back they told me they wouldn't buy it because they were getting a new edition next year. I DON'T THINK SO. New edition? Puh-lease....it's not as if the composition of the human body has changed, and it's not like there were any exercises in the book.
Anyway, on the whole, I used to get back a nice enough price to make it worth it for me. Otherwise, if they didn't offer a good price on a particular book, I just took it home and kept it for reference.
My last year or so, the CSULB bookstore responded to student complaints about textbook prices and buyback rates. So, the bookstore started guaranteeing a minimum 50% buyback price on certain texts. So, when you went in at the beginning of the term and saw that your textbook had a "50% back guaranteed" sticker on it, you could feel more confident about buying that particular text.
.....Kelly