if you have the proof of delivery and buyer's data. you are safe.
send a Letter to excise office with copies of both mentioning date of sale. Inform that the vehicle is still running under your name, attach the traffic violation ticket copies you receive. Request that the vehicle shall be blocked immediately. Send by TCS so you have proof of delivery.
Send copy of the same letter to original buyer. as well, wait till he turn up walking on knees.