If you are in the United States and want to save money on textbooks, the best 'legal' approach I know is the purchase of international text books. These are texts that are printed for use in countries such as India, and that are then resold into the U.S. market. According to Time.com:
http://www.time.com/time/nation/arti...838849,00.html
these texts are legal for you to buy even though they may not be legal for the seller to sell.
Anyway, I've heard students who bought $150 textbooks for under $30. And the books were the same as the US editions, just with different covers. Of course, that is your risk -- ie you must trust/hope the editions are the same.