Banning account trades in the forum might finish the deals happening in public, like in Auctions or Digital Goods sections, but they will start doing it in private
I hate that argument with a passion. It's an excuse to do nothing in the face of wrongdoing, and such poor reasoning would never fly anywhere in our world of laws.
But on the other hand some guys buy high rank accounts through money just only for sign campaign.
I'm concerned about this, of course, but I'm far more concerned about scammers buying an account with green trust and high rank. Those sorts of account sales can do a LOT of damage. Fortunately when this happens there are usually signs that the account has been sold (change in posting habits, waking up after a long time, change in password/e-mail, and attempting to do a lot of deals at once), but you can't count on those warning signs existing.