This question is for anyone who has sold there used books back to School?

Ya know how expensive our books are right? Well I'm trying to get a little bit of money back and I spent about $800.00 alone on my Summer Books well I'm just wondering if I"m going about it the right way, is that to go through the University and do it through them, which by the way there offering me I can't even believe it $14.00 a book, lol which kinda freaked me out, My question being, is there another way of selling your books? Can you post them on a website through the school, anything, I'm just looking for any suggestions, and when I put in all the ISBN #'s they only wanted a couple the other ones they didn't even want, Do they seriously change books every single semester? I know I'm knew at going back to school, I can't believe things have changed this much? $800.00 for books, just to be tossed out at the end of the semester? It seems ridiculous, to me, any help will be surely appreciated, Thank you