I think the only difference I see is that with TF2, you have dedicated servers and there are the "official" servers, but that's not the only way to play. I think I would be less inclined to cheat if I had the chance of not being able to play at all. Plus, you would have to spend $60 on a new game for a new account unlike TF2. On the other hand, I did occasionally see some botting in WoW and people flying through the map automining and gathering nodes.Selling the gold directly benefits Blizzard and the people who buy items but doesn't benefit the rest of us who play the game normally.
Making real money trading official is going to encourage more botting. Using TF2 as an example, there are people out there with huge idling rings.
Now instead of cosmetic items, imagine people with $20,000 worth of items (like mattie in TF2) tied directly to your enjoyment and power in the game.