First course: Check with the concerned RTO, whose name the vehicle stands registered as on date. If it is in your name, give a written application to RTO that you have sold the vehicle to "Mr.X" hence the ownership may be recorded as transferred. Get acknowledgment of this letter duly received by RTO for future records. If this course does not satisfy you, adopt the second course.

Second course: Contact the person whom you sold the vehicle and get the chain of buyers. then convince the last man to get the ownership transferred in RTO records, if does not cooperate, lodge an FIR that your vehicle has been forcibly taken away by the first buyer and give names of all persons involved including first and last buyer. They will then cooperate to get the ownership changed in RTO records.

You may inform the concerned Registration Authority by sending a registered letter, with a copy to the local police.