Personally, as a WOW player, I kinda wish Blizzard had started to sell gold themselves a long time ago, as I'm sick of the badly spelled scam whispers, emails, websites and spammy messages in the cities of the game.
However, I can foresee one big problem.
Anyone who's played WOW or even more so, diablo 2, knows there's a fairly sizable proportion of the player base who's more than happy to screw over an entire group after a few hours play just to steal an item they don't really need.
How much worse is this going to be if it's worth $10 or more? how often are you going to see an epic drop, know it's worth about $150 at the Auction House and see the whole group going 'no, no, you take it, I don't need it, it doesn't suit my class or playstyle'.
I just foresee it getting a lot harder to be the nice guy, hen it's costing you real money each time.