No, you're wrong, Blizzard stop banning gold buyers long time ago, it's now become a fact. The reason behind is because Blizzard get tired with all the complaints for people got banned by mistake from that action. Tell me what happen when someone hates you send 10K gold to your mailbox, or a person give away his gold to hundred of people, will Blizzard ban you and all of these gold receivers ? No, never. Reality speaking, Blizzard don't have enough evidences to ban someone just because he receives a large amount of gold. By accident ? by give-away ? bad intention ? These are too many reasons they can use against Blizzard. Moreover, the loss-n-loss mentality (lose money to buy gold and lose account after that) of these banned gold buyers will usually lead them to flood the Customer Service forums with angry complaints. One more note, gold buyers are usually rich customers. Do Blizzard want to upset their rich customers ? No way. So they take the safer approach.
What happen to a banned gold seller ? He will just shut his mouth up, because anyway, he gains something, and because he feels guilty. Moreover, it's more difficult to explain why he give a large amount of gold to others. He will just buy a new set of CDKeys and try again. Who benefits ? Blizzard.