@Tech brother, the best thing to do is to visit the registration office, transfer the car into purchasers' name yourself. My friend is a lawyer and he advised me to always to that to aviod any legal complications which might arise after the sale of a used car.
I think any genuine buyer would not mind if you tell him that you will deal in this way.
In my case, the buyer asked me to go with him to reg office and I said welcome and transfer was made in half-an-hour. Now I have peace of mind that, I am no more responsible for the car.
Or you can make a sales agreement on a stamp paper. Stamp vendors, oath commissioners or notary public have the texts of such agreements in their PCs. The agreement clearly mentions that the seller would not be responsible for any legal complications, damages arising out of the use of car after the sale and handover of the vehicle to purchaer. This paper has some legal value, otherwise, a simple transfer or open letter is not enough, if the sold vehicle comes in conflict with the law of the land.