Buy and sell digital products reccomendation

Ok, so my step-son wants business website that will allow users to register and then buy and sell digital products. The products he wants people to list are items from games he plays like Counter Strike, Team Fortress etc.

Can you suggest what plugins i need for this? MarketPress? Classifieds maybe? i've been trying them out but not really have much luck with the functionality requires.

Let me know what you think.