That's the thing, though. It can and has happened before - through paypal etc. What makes it "illegal" or "black market" trading is the fact that no company wants to put out a game and then be left out of the profits to be made. You can introduce decentralized anonymized currency transactions all you want, at the end of the day the company controlling the servers isn't just the landlord, they're fucking god in their digital world. And you better believe they'll be taking their rent.