Can the moderators do something? Look at an IP log, confirm if he really sold the account. Did the account password recovery email changed lately? It would be so much easier if account trades are banned, to prevent such discussions.

I hope that he will return the funds to all people that have an unfinished deal with him. If he always could sign a message the good way, and now they are bad, then it is possible his account has been sold. But he also could have faked it, we only can know this if the MODS/ADMIN can help us to solve this.

Or ask some very good detective(QuickSeller???)