Ebay without a doubt. I started looking as soon as I got the acceptance letter. I have gotten some unbelievable deals. It is best to leave enough time to be patient. I am already looking for second semester books.
i use amazon.. but only for books that really save me some money. I bought one of my books at the book exchange today because it was $2 more than the one on amazon and i could see it and knew it was in great condition. Just depends on the book and the deal i guess.
Amazon.com's new/used third party sellers. Often way cheaper than amazon itself for still new or barely-used books. I got an absurd number, like five, of Step 1 books (HY, BRS, etc) there a while back for $30.
If your school is anything like mine, you will be bombarded with e-mails sent out to class lists from upperclassmen trying to sell their books. Annoying, but a good way to pick up cheap books
My friend gave me this amazing website, www.addall.com, where you plug in the book name (or better yet, ISBN number), and it searches all online book sellers for the best prices TAKING SHIPPING COST INTO ACCOUNT. It sometimes even gives you coupon codes so you don't have to search retailmenot.com.
I have found that Alibris also has very competitive prices and is sometimes cheaper than Amazon. But I really recommend addall.com. Too bad they only provide up-to-date pricing on books...