I’ve just finished adding two new vendors to the site, bringing to total number of searchable sites up to 43. Moola4Books is a company based out of Fort Wayne, Indiana that has been buying and selling books since 2008. The initial pricing that I’ve see looks like these guys have pretty competitive pricing.
Buyback101.com is based out of Florida and specializes in K-12 books, so they may buy some of those books in quantities that other textbook companies won’t.
I’ve had a good amount of communication with both companies prior to getting them enabled today. Both seem very eager to provide excellent customer service and to provide accurate and prompt payments. I’d encourage you to give them a try when you see the opportunity. And please remember to leave feedback once you have completed a transaction or two.