This site is like steam except it allows you to trade in certain games. Plus they're having a sale so you can pick up GTA III for 2.50. I got Bully: Scholarship Edition for 10 euro aswell. It's a pretty cool site I suggest you check it out
P.S : Despite the domain name it's not just for UK. I'm from Ireland and it works just fine and some of the games are even advertised in US Dollars.