Not necessarily bitcoin address. If he posted an altcoin address and he can signed I think that could be enough.
Oh really? I'm not into altcoins so I didn't knew about that, but 99% of the users here prefer signing a message with a bitcoin address when selling or proving ownership of the account.
Anyways it would be a good alternative in case he posted one altcoin address and never posted a bitcoin address.
Yes a good alternative way to prove. But if he nevrr post bitcoin addrerss and this account os solely for selling purpose and he never post altcoin address. What should he do?