Amazon lists books at their store price and used price so you can decide whether the used seller is giving you a good deal. You can even qualify for free shipping if you are a student with a college email address. The Amazon Student program lets students get free two-day shipping for a full year.
eBay lets sellers put their book up for a bid, which means you might score a book for really cheap if no one else wants it. If it is a book that is in high demand, you might have to shell out a few extra dollars. You can also find collectible, rare edition books that are great for book collectors.
Buy or rent used textbooks that are in excellent condition from Valore Books. The website specializes in college textbooks, which means you can typically find the edition you need for class. The rental program is interesting because you even get free shipping when you mail the books back.