Buyer of 2 wheeler not transferring ownership to his name

I exchanged a 100 cc scooter (TVS 1999 model) in 2009 at a bike dealer. The new vehicle was registered in my Nephews name. The invoice shows the exchange amount as amount due from me. I was given a delivery note signed by a person along with a copy of his driving license. I thought every thing is fine but in 2012 I received a notice by post from the transport department for traffic violation. I checked from the website of the transport department and the ownership is still in my name and the person is not doing any thing. What can I do now?