Well, on a purely legal sense (which is the only one that I would consider binding, as it really IS the only sense that is binding)...
Buying Gold online is a breach of the WoW End-User License agreement, which forbids the buying or selling of any in-game item (including gold) for real money, as you do not really 'own' WoW... you are only renting it from Blizzard. In the U.S.A., selling WoW gold actually constitutes a (albeit highly contestable) form of fraud.
It definitely constitutes Breach of Contract, so Blizzard is completely in their rights to nuke your account over it.
On the contrary side, I don't think that Blizzard can stop you from handing someone a stack of money, even if they for some reason have just mailed you a bunch of gold. The law/EULA is rather uninforcible.
So, how should we then live? It's a good question.
Personally, I would avoid buying gold, as we are supposed to be 'salt and light', and therefore engaging in behavior that Blizzard considers wrong is just bad witness.
Please note that I started that sentence with Personally,. Disagreeing with me will not result in ostracism.

But I hope that you chat it up with other believers and reach a decision. Remember, we are not bound by law (W00T!!), but we ARE out to change a world here, and so sometimes that means we have to make some sacrifises.
Signed,
The guy who will never actually get enough Gold to buy and epic mount, but I am cool with that.