You could eventually sign a message from your old address to remove any possible doubt (if you have access to any of the addresses that you used in the past) that account changed hands.
To sign a message with bitcoin address can only help to know if an account is not compromise, but not useful if a farmed or old account is sold. I can build an account, stake an address with it on this forum, sell the account, give the wallet to the person I sold it, tell him how I have helped him to sign a message in the past and how he can reuse the address to sign another message, old account can be sold too like that.