It is a textbook comparison site, as far as I can tell. All I know is, I saved about $300 on my textbooks! Nice.
Looks like all it does is search Abebooks and most likely gets as referral fee too. I'd use BookFinder.com instead: http://www.bookfinder.com
I buy three times a year. I saved $300 this semester, so by my math - that means I save about $900/year. Pretty useful.