Try "alibris.com". They are a website/network of booksellers and you can compare different sellers' prices. (Btw, when I compared Amazon's prices to Alibris, Alibris won every time.) My hubby got a textbook there, "new" it was around $100, barely-used it was around $50. If condition is important to you, pay attention to the descriptions. I've bought used books off of their website, they have some great deals.
agnes!