AFAIK, they're not ban-able but they can be painted with red trust. You can just go to Meta to see it for yourself. The only possible way to ban account is "plagiarising".
You can get banned for things other than plagiarism. Constant spam, consecuposting, and ban evasion will all earn you bans. You won't get banned for account sales, but you'll probably get red-tagged by a DT member. As far as merit sales, you'll definitely get tagged if caught, but I don't think I've seen anyone banned for that--and I can't even remember if there's a hard rule about it.
People selling accounts are ballsy here, but they have to sneakier about the merit sales. I guess deals could be done all through PMs and such, but I haven't seen any merit sales threads around here. I think all that happens on telegram.
The good thing is that bitcointalk has a lot of people with very sharp eyes looking at merit transactions, so a lot fewer people get away with that crap than you'd think.