You could try www.half.com
it is by the same people that own as ebay but for books, games, and media type stuff. The only difference is that it isn't a bidding thing you see the price and the shipping is a fixed amount but you still use your ebay account. I get most of my school books on there and you can find games on there for pretty good prices too...
The best thing is when alot of people are trying to sell the same game quick they will under bid the other people to get theirs listed first so sometimes you can get games for dirt cheap. But sometimes the games are pretty much on the same level as ebay but its worth a shot.
**Edit** I just actually looked on there and the best price for COD4: modern warfare for PS3 is 35. Sorry not really any different but you could always check on some other games you may be looking for and you might find em for pretty cheap