I want to post about a great site I've been using for a couple of years - PaperbackSwap.com. It's not only paperbacks, but also textbooks, audio books and curricula, plus more!
http://www.paperbackswap.comI'm "JCCrooks/J.C." there. If you sign up, please use my e-mail as the referring member -
4crooks@comcast.net. Then make sure to tell others so you can get referral credits, too! I'll add you to my "friend list" and you'll get notice when I have deals on my books!
There's a great HS'ing forum at PBS, too. If you subscribe to the red "sticky thread", you'll get e-mail notices when someone posts a deal. I do the same on the "teacher's corner" forum, too. It's not nearly as busy as the HS'ing forum.

There are two sister sites, too - Swap A CD and Swap a DVD. I've received lots of educational stuff on both - especially Standard Deviants on DVD. You can swap your credits between the three sites.
http://www.swapacd.comhttp://www.swapadvd.com