None of your mentioned scenarios would imply that he is untrustworthy.

Scenario 1 does not imply the he would do 'evil' for 0.3 BTC. That is only your assumption. It is based on nothing except for the fact that he put his acc for sale for 0.3 BTC. And this was 3 years ago.


Scenario 2 does not imply this either.
Even if the account has been sold, the person who owns this account has proven to be trustworthy now.


I don't see a reason why tagging would be necessary.
I don't think it would help to prevent scam in any way. Neither would the community benefit from it all.

If he wouldn't have shown that he is trustworthy, i would agree with you. Warning others would be appropriate then.
But in this case, i don't think so.