Diablo III has an auction house system that lets players buy and sell items for real money or in-game gold.  Former Diablo III game director Jay Wilson sasy he thought that  it would cut down on fraud and black market sales and only be used by a minority of players.

Wilson says that this estimate was far off from reality.  He noted that a vast majority of users tried their hand at the auction house at least once and half of them use it on a regular basis.

This disparity "damaged item rewards", according to Wilson.  He also says that the focus on money has detracted from general enjoyment of the game's campaign.

"I think we would turn it off if we could," he said according to Joystiq, but noted that Blizzard is reluctant to do so for fear of upsetting the game's current user base.