I've saved a lot of money by using Ebay.
You can be pretty much anything you can think of on Ebay. Some items you can just buy right away, and others need to be bid on. The bidding is really fun, especially when you're watching the countdown clock near zero. I have been able to get any Playstation 3 game that I wanted for under $15. If I had gone to a store that sold used games, I would have spent a lot more.