While I do not endorse account selling, it is not against the forum rules. Using deceptive tactics to destroy the property of others is untrustworthy behavior. Mind your own business. Flag supported.
Even though the account he was selling didn't belong to him in the first place?
If that is the case, and the original owner wants to make a flag and has evidence I would support that as well. "what if" games do not justify this behavior.