Rumor is, some MMO games have markets exceeding GDPs of plenty of first-world countries, and ingame items are used by gangs to move funds across borders. Both Cory Doctorow and Neil Stephenson wrote books featuring this phenomenon, and I'm pretty sure they both usually take their ideas from reality.
Since Steam is a Big Guy, and its market is dedicated to this very activity and sits on top of many games at once, I'd guess it to have a sizeable slice.