Your college might have a Facebook page for students to buy and sell textbooks on. Mine does, and I got a couple there for really cheap. I also got a lightly used iClicker, which was great since I only spent $15 instead of $50, which is what it costs new from our bookstore.
Also- check the price for the textbook one edition older than the current one. I mean, ask your professor first of its ok. I did for my chemistry class this year, and spent $6 on the 2nd edition on Amazon rather than $130 on the 3rd edition 