What happens if you sell it yourself and then you try to lock the account because you still have access to everything including email because you don't really want to lose the account? Then who should we believe in each other's claims?
I believe if you are the account owner then you have access to the email address, wallet address and it is very possible to get you signed messages with it. But just one thing, what if it's actually you who sold the account and then you lock the account by resetting the password with a secret question?
IMO, only main users know about secret questions.
Then I do accept the consequence that my account will be locked forever or ban or red trust due to selling account. I guess I do not have any proof that I am not selling that account. I just suddenly came back to Bitcointalk and saw my account being used and I want to take it back.
will this be viable proof for ownership?
I don't think it does. Besides the reason you pointed out, I believe only Bitcoin signed messages are accepted for account recovery and doesn't include addresses from other networks.
Let the admin decide which can be used. But that is my only address on the forum.