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This is the best way you can do for now. You can only get little amount if you have no signed message. One more thing, saying this things looks like you are in a hurry selling this account. Are you sure this isn't hacked?
It's not like he'd admit that the account is hacked.
judging by how op is avoiding the question, id say its likely that the account is hacked.
Which question I`m avoiding?
The question about whether you can provide a signed message from an address that was posted by the account you are selling at least 3 to 6 months ago... It's even better if the address was quoted by somebody, so that we know you didn't just edit the post and add in your new address after you acquired the account...
If the account turns out to be hacked, there's a chance it will get negative rep, and the buyer will most probably lose their 0.16 BTC