CS:GOX
Provide the ability for Counter Strike Global Offensive players to sell their inventory for real cash - while simultanously creating a market place to purchase inventory for a cheaper price than the Steam market.
Counter Strike is a game played on Steam. It's extremely popular and the ability to buy custom skinned weapons is a big part of it. These skins can cost as much as $800 to buy. Problem is, you can never sell the weapon you win / buy for real money.