Sir how about if i want to buy account with sign message how to do?
My suggestion would be to use escrow. Most sellers will not tell you the exact name of the account in advance. Thus the escrow would have to check for the oldest available address of the account and request a signed message for you.
Like I have linked an old post in my 1st post here. The address posted in Dec 2013 is still under my control. If I wanted to sell my account I could sign a message with this address saying that I freely sold it to you. That way you can proof that I indeed sold the account to you and you are now the rightful owner of the account. You dont have to reveal this signed message unless you need to. As long as nobody questions whether you are the rightful owner of the account shorena you can keep the signed message somewhere safe. Requesting a signed message should be normal for a good escrow service, but it might not always be possible.
Not everyone posted their address in the past. If there is no old address you can still request a message signed with the payment address. E.g. you buy my account and I want the payment on address 1sho... I could sign a message with 1sho... stating that I just sold the account shorena to you.
Once you have control over an account you should stake one of your addresses in this thread by tomatocage ->
https://bitcointalksearch.org/topic/stake-your-bitcoin-address-here-996318This enables you to sign a message in the future should you want to sell the account or if there are other issues where you need to convince someone that you are the rightful owner.