I found out about two sites that I SO wish I'd known about when I went back to school for my Specialist degree last year! I thought I'd share them with you in case you were heading back to college... or maybe you've got kids going to college! The first is Campus Book Rentals.
But, for those of you that didn't rent, and didn't sell your books back- here's a deal for you! YOU can rent your books out to other students and make at least two times as much as you would just outright selling them back to the bookstore! It's called Rent Back!
Hope this helps you out! Now, go clean out your bookshelves! :)