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.
Imposing a rule without a viable way to enforce it wouldn't do much good.
If account sales are banned we're gonna catch a few idiots who attempt to do it in the open and that's about it. I don't see what we could possibly do about sales happening outside of the forum. There are plenty of examples like that IRL too, e.g. Chicago gun restrictions rendered largely useless by Indiana. We can impose such restrictions to make ourselves feel better but I doubt it would make a dent in the actual problem.