Hi,

A good way to spare some money is buying your books on auctions sites (Ebay, Overstock...)
Please note you can also find new stuff out there. 
You are responsible for the price :-) 
You simply bid!

The following site is a good starting point if you are new to auctions;

http://auctions.woohoow.net

Contains links to the most popular auctions sites, tools, information, tips...


For those who will give it a try: good luck!