I am not sure about buy/sell but few people may take chance if they can make profit before being tagged.
I never really had a good sense of what the account market was like in terms of prices for the various ranks (and including green trust, number of merits, etc.), but I would imagine that a Hero or Legendary account would sell for at least a few hundred dollars if it wasn't red-trusted. If that were true, I doubt many people would take a chance if there was even the slightest possibility of it getting tagged in the future.
On the other hand, I'm not so sure about bounty hunters. Lately I've gotten the impression that bounty managers don't really care if their participants have red trust or not--not surprising, given that a lot of the projects being advertised are either crap designed to make the devs quick money or flat-out scams. So I'd imagine there are probably some people who'd take a chance that their bought account is going to get tagged. They'd also be the ones who'd buy an account that's already been tagged.
Regardless, account sales are still bad no matter how you slice it. I'm betting most of them happen away from the forum, too.
the main aim is to make sure the warnings are visible to some guest users or newbies who may not be aware that buying accounts here isn't something cool
Spot-on.