99gamers

99Gamers started on reddit out of a couple guys sheer frustration of the used game market. It's been really awesome to watch them take off. I've done 5-10 trades myself, and it's normally a great experience.

Will be interesting to see how much market is there with console games and the rise of mobile and MMOG's

This is really an interesting concept, especially given the recent developments in regards to DRM (a reversed product decision that still is haunting Xbox One sales) and brick and mortar retailers having a stronghold on pricing, distribution, etc. in this market (giant source of contention with game publishers). Will keep an eye on for sure and maybe even use soon.

How will fraud be monitored? With digital distribution codes, etc...

I launched a similar service to this, http://simplegameswap.com. It's interesting to see where users gravitate. I still end up using reddit.com/r/gameswap because of the massive user-base.