Amazon Used Book Sellers

If you have a tight budget this could be your answer. The used book sellers on Amazon can pretty much be relied upon to ship within a day or two and be very fair in their dealings. It is important to them to give you good service and a good product in order to keep their ratings up. Individual merchants have their own return policies, etc - you can look them up by clicking on the merchant. I consider it to be a reasonably reliable way to purchase like-new books for an (often) tiny fraction of the regular price, even when you consider shipping. I've purchased new books off the shelf that were in worse shape than any I've ordered from Amazon.

Used books help preserve the environment!

Amazon is my personal favorite but these are also often recommended:

alibris.com
abebooks.com
http://www.barnesandnoble.com/used/search.asp
bookbyte.com -can be a good place to find deals on expensive books