To clarify a bit; They are asking you to prove not only that you own that particular bitcoin address, but also prove ownership of the forum account you are posting from.
So what you need to do is find a bitcoin address you posted in some previous post, and then sign a message using the described method user above quoted posted in link.
That way, they will know you are the true owner of the account, which means;
-that the account wasnt hacked
-that if you give control over the account, no one can claim it later on under some excuse
This method is highly needed when dealing with forum account trading, and/or dealing with loans.
I see you were/are buying an account -
don't accept/pay for an account where the seller can't/won't sign address from unedited older post made!cheers