based on my ltd. knowledge; If you had provided everything to the buyer and were fully willing to help him in the transfer of ownership in his name, buyer also agreed, but then never returned, then you may visit the relevant Excise Office, and ask for the blockage of transfer of ownership, and may be even cancellation of registration.
On parallel grounds, you may also ask the Police department for confiscation / impounding of your vehicle, on security grounds, and that vehicle may be used in any malicious activity.