Bottom line? It's not your problem. They may trace the car back to you but all you need is to provide the name of the person you sold it to. The person driving the car is the one with the problem. They have to prove they did not steal it and explain why they are driving an unregistered and possibly uninsured motor vehicle.
When you sold it did you make a bill of sale and keep a copy? Did you keep your tags? If not I don't know, if so you have proof to show the cops and can keep yourself out of it. Unless you have a witness or can can a copy of a canceled check, I don't know what will happen.