Purchasing/selling ingame gold or items for real world money is a violation of Blizzards TOS. You’ve agreed to this the very first time you logged in and the first time you login after every patch. Your account will indeed be banned.

They have routines setup to flag certain activities that occur in the game and investigate them. In this case they will check where the gold came from and follow the trail. When it comes from known characters that facilitate this kind of transaction, the accounts involved are banned.

And as an extra added bonus, the sites that advertise this kind of thing are generally loaded with keyloggers. Just visiting one of the sites could cause you to lose your account to hackers.

If you value your account, I would refrain from this activity. You might also read this statement from Blizzard:

And as one of these repliers said you should watch out if you do buy gold. I have heard all sorts of tales about how people got themselves banned, got keylogged and had their accounts ripped off, etc., etc..

Buying gold isn’t a wise move, nor is it necessary with all the dailies that are available too (if your toon is 70 of course).

Also, gold seller sites are about as legitimate as a three dollar bill. Even if they don’t take your money, and you do get the gold you ordered, AND you don’t get banned (and you will, blizzard monitors every gold transaction over a certain amount). There is still the chance you could get infected with a key logger from their site.
If you really care about loosing your toon, dont do it!