Besides, bob didn't use his account for this, so the "warning" wouldn't help other account sellers. Just like account sellers who hide behind a Newbie account.
The flag (once created) would warn anyone considering to trade with Bob that he previously broke an agreement, and has not made the other person whole.
The accounts didn't make high quality posts OP, so bob didn't break your agreement.
I don't see anywhere in which it was stipulated the accounts made high quality posts.
https://web.archive.org/web/20190619113422/https://bitcointalk.org/index.php?topic=5155184.0 <-- OP's sales thread
http://archive.is/rlBTD <-- telegram conversation.
Anyone can create a flag, but I don't think you'll find support for it.
Unless you can explain why there was not a broken agreement (you say the accounts didn't make high quality posts, but this was not a term of the deal, implied or otherwise), if you are not supporting the flag, you are giving people an excuse to not honor their agreements with those who are unpopular.
If you say you are going to do something, you need to do it, and if you don't, you should be held accountable. There are no
ifs, thans, or buts to this concept.