Why have I received a violation ticket for the vehicle I have already sold?

Notices are generated by computers picking up address from transport department records. The very fact you have received a notice means that vehicle continues to be on your name in the records even though you might have sold it. Selling the vehicle without insisting on change of ownership is a very dangerous practice. Signing a delivery note is no guarantee of change of ownership. You have to insist on buyer registering the vehicle in his name, failing which you have to get it done yourself at RTO. Unless you do so, you will continue to receive violation notices. Still worse can happen if the vehicle is involved in a fatal accident or terrorist activity. You will have to suffer the consequences.