The sole purpose of this account since it has green trust will be to scam if dropped into the wrong hands. Although you may get more what it's worth, do the forum a favor and ask the guys who left you trusted feedback to have it removed.

Yes he means signing a message (which includes current date and probably the name of the buyer of the account) signed with a bitcoin address published by that account several months ago.
You don't need to publish that message now, just let us know if you do have access to a private key to sign a message later when the deal is closed.

Can you provide a signed message for prove the ownership of the account? A simple request, thanks for the attention.