If you cannot sign a message for any reason then there is going to be a risk to the buyer that the account is hacked.
Sorry to butt into this thread, but can someone explain how signing the message from the account would verify that OP was the original owner? Is there anything stopping him from changing the bitcoin address on the account to his address, and then signing from that, if the account was hacked? (not accusing you of anything OP, just curious on how this proves the original account owner)
Say for example that an account posts a specific address on Jan 1 2015, and the post is never edited. It would be reasonable to believe that the owner of the account controlled the private keys to that address when the address was posted. Therefore as long as the post is not edited you can reasonably say that the account has not been hacked since Jan 1 if they can sign a message from that address. With a lot of time passing and a lot of posts being made since such post had been made, then it would be reasonable to say that the account was not hacked.
It would be possible for the account to have been sold by then as the seller could include the private keys with the account when it is sold, but buying a sold account really is not a big deal (as long as it is not hacked)
