www.half.com

Run by Ebay, this amazing site lets you buy books (used or new) for much lower prices than you can at, say, campus bookstores... You can search by title, author, or ISBN, so it's easy to find the exact book (and edition of said book) that you need. I bought all of my books for this term for $70 or less. :)

Selling books is also pretty easy. If you already have an Ebay account, you're all set. I use PayPal which keeps all payments to and from my bank account automatic. You can set your own price, and choose from a few different levels of wear on your book (ranging from "Brand New" to "Acceptable"). Depending on which level of used-ness you set your book as, it will suggest a different price, as well as showing you the averages and last sold for prices of the same book. If your book is a new edition, which it probably is if you used it for a course last term, then you should be able to get over $50 for it. (I actually set a price of $80 for my Psychology text book, and it is now one of the more reasonably priced ones on the site.)

The next couple of weeks are the best time to put books up on half.com, because the term is just beginning at most schools. (More demand = more money for your old books.)

I'd definitely recommend giving this website a shot; you'll make much more selling your used books, and get newish books in great condition for far less than you could in most other places.

Spread the word!