How can you prove that someone recovered an account?

Hey I was scammed for an account a few months ago and I didn't post a scammer report because the guy who sold it was already banned. However, the guy is now unbanned and back on sythe making trades...

Now, I have proof that he sold me the account. But how can I prove that I am no longer able to log in and that the info he gave me is fake? I cant think of any ways...