Please don't think you hijacked anything, I'm delighted you were inspired to start this thread! BTW, I've also used Amazon, ebay and half.com for textbooks but am always looking for better sites.
And if you're an AAdvantage frequent flier member, don't forget to link through their website to the merchant (list includes Apple and Overstock) when you're ready to order, and get more miles per dollar spent.
http://www.aadvantageeshopping.com/allmerchants.jsp