Did you get some affidavit signed by the buyer and you? Something which says that the car is now no longer your responsibility and is the responsibility of the new buyer now?
There are a number of possible options to explore. If I were you, I 'd contact the buyer, and ask him about this. If you are unable to contact the buyer anymore, then you might want to report this to the police. Get an FIR made in this regard. If it causes inconvenience to the buyer, he deserves it for being a lazy ***. Seeing the current security situation in Pakistan, I would advise you NOT TO TAKE THIS LIGHTLY AT ALL! God Forbid, tomorrow the car is used for any misdemeanor or worse - a big felony - you will be held liable and it's better to take some pain and get this reported to the authorities - make sure you get as much documented proof that you sold the car and that you reported the matter to the authorities as well.
Best of luck!