Oh gawd, Ratimov related drama keep going on...
You can't prove it directly. He caught sellers and knows all the nuances.
He maybe knows all the nuances but if what you are saying is true and another person took control of his account, that person will most likely slip, sooner or later.
Of course, this is not strong evidence, so I created a topic to attract the attention of more experienced users
Since he is currently only active in your local board, none of us here who don't speak Russian can help you much. What's the opinion of other members of your local board, do others have the same suspicion that he sold his account?