OP, what do you suspect from the account? just because it has been inactive for a long time then change your email and password? yes, maybe this could be an indication of the account changing hands. either from hacking or trading.
but if you do it based solely on that evidence, I'm afraid you'll get more accounts in your accusations.
if you have evidence of the trade conversation or the account being sold via social media. you can continue. if you don't have it, it's better to stop.