Well, you were driving an unregistered vehicle with tags belonging on another vehicle. I really do not think you have any legal basis to fight this, don't get your expectations raised. Also, don't drive unregistered vehicles.

With that said, let's talk about the status of things. You were driving an unregistered vehicle. Was this vehicle ever registered to you before? If it wasn't, then go to the DMV and get a copy of your abstract. If the car was not registered to you at the time of the ticketing, your abstract will prove that.

Bringing that to court, you can present the abstract, explain you were trying to buy the car, show a bill,of sale hopefully dated after this incident, the vehicle registration and insurance stuff proving the vehicle is taken care of.

Technically you are not responsible for the state of a vehicle you do not own. If you can prove that the sale of the vehicle occurred after this incident and that the vehicle had never been registered to you, you should be excused in the interest of justice.

But don't drive unregistered vehicles. It's a great way of fucking yourself over for years to come.