Blizzard has launched WoW Tokens, the system that will allow players to sell gold and game time to each other.
If you have in-game gold, you can buy a token from the auction house for 30 days game time. On the other hand, if you have all the time to play but need gold, you can buy a token with real money, and then sell it in the auction house for gold.
In fact, other MMOs already have a system in place like this one, so it should be interesting to see if Blizzard sees favorable results for the aging, but still popular World of Warcraft. You can read more about it here.