Is tagging these accounts for being sold the correct move? They have 0 positive trust and are not trying to insert themselves into any prestigious positions in the community. I think neutral would suffice. If they had some green trust I would want to be a little more aggressive on tagging.
In my opinion, I believe identifying and tagging them whether on zero positive trust or not is still the best way forward because the new person behind the account isn't the original one. If the person who purchased the good account had any good intentions in the forum from the start, then they would just create a new account, beginning that long journey in the forum like most of us.
In short, tagging or discouraging account sales;
1. Reduces account theft and hacking.
2. Minimizes bounty and signature campaign cheating
3. Minimizes account farming
A good example of why account sales or change of ownerships should highly be discourage is this loan fraud case -
https://bitcointalk.org/index.php?topic=5521225.0