I see your point, Zin, but I really think Blizzard is bright enough to tell the difference and ban from the sales end, not how they made the gold. Consider this: how do farmers sell gold?

Many large sales from one account (to wit: changing names does NOT fool blizzard's per-account logs).
Many small sales from one account.
Many small sales from multiple accounts tied to a specific owner.
Sales via trial accounts.
Many small sales from multiple accounts to a 3rd party recipient.

All of those transactions are now tracked. So they can link all the sales on an account and what account(s) those "mules" got the gold they sold from, so they can chain-ban everyone from the seller all the way back up through the farmers that supplied 'em.

People make the silly statement that Blizzard cannot track this stuff and that's why gold sellers still exist. I think it's less that than it is just plain resource-consuming to prosecute the thousands of these folks that do it. There's a breakover point where taking the time to investigate and ban accounts takes up more monetary resources from service reps than you lose to gold sellers' negative effects. I haven't the foggiest idea where it is but I'd bet you a week's pay Blizzard knows.

[edit] Also, don't forget that a farmer that's smart enough to multibox effectively will require higher pay than his button-mashing friends.

Additionally, don't forget that multiboxing makes you a very OBVIOUS target for reporting to GMs. Which means you have a much higher chance of being investigated and, if you're a farmer, multi-banned because they chanced to catch on to your black-market dealings. I would not underestimate this aspect.