I see a lot of users are selling forum accounts but they are unable to provide a valid signed message
don't buy such accounts without a signed message,if you are buying such accounts then you are taking risk of losing money/account nd reputation also encouraging them to get more hacked accounts and sell them(if they are real owners and legit sellers they can sign a message without any problem)
I am the legal owner of the account that is sold, I created it. but if there are no messages with bitcoin address the fact that I can sign in your opinion? and as mentioned escrow preferably
If you cannot prove the ownership of account from a posted bitcoin address then in my honest opinion it will be difficult for you to sell the account, because you (or the real owner) can return one day and pretend to get back the account.