If you're interested in older games, I highly recommend Goozex. I've been using that service for a couple of years with great success. It's a game trading sight. Games are given a 'points' value based on supply/demand. So, a brand new game is usually 1,000 points, while a 3 or 4 year old title might be worth 150 points. The only money exchanged is you pay Goozex $1 per trade that you receive (i.e. you don't pay a fee when you send a game) and you pay for shipping of games you send (which costs me less than $2 sending via regular mail.)
It's great for getting games that have been out a while. I wouldn't use it to get the newest online game that I want to play on day 1, but for an RPG that I can wait a few months? It's a great way to go.