Today’s generation just has to type in a URL and hey presto, there are more than a dozen sites plying cheap textbooks for sale or rent. Most of these sites are US only.
http://www.makeuseof.com/tag/sites-college-textbooks-cheap-rent-buy/
DawsonITE is a blog devoted to Educational Technology. It's compiled by Rafael Scapin, Coordinator of Educational Technology at Dawson College in Montreal (Canada).