The buyer cannot take possession of the property without getting the property registered to his name.
If he is making an illegal entry to take possession, you may approach police, and if the police is not cooperating then you may approach the top police officer of the city/district for intervention and relief.
If the police is not cooperating then you may approach judicial magistrate court with a petition under section 156(3) cr.p.c. seeking direction to police to take proper legal action on your complaint.
You may even approach senior xcitizens welfare tribunal with a complaint against the illegal activities of the buyer and his threats posed on you.