Your account is probably worth around 0.1BTC anyway alone.
I doubt that. Account prices have fallen a lot recently.
(EDIT: I will delete this post if you could sign an address that was posted by the account before November 2015)!
Private keys can be sold along with accounts, so a signed message will not necessarily mean an account was not sold.
Oh, I was going off an optimistic account price.
I assume as you are in this market much more then you will know the greatest price to ue.
Private keys can be sold and other online accounts can be sold too so signed messages from other accounts are easy to do. However, it is less likely that will be done!