I’m guessing that you didn’t take the plates, then, and he drove off with your plates on the vehicle? That could be a problem.
In most of the states that I’ve lived in, when you sell the car you take back your plates and take them to the DMV, where they will cancel your registration. (If the new owner goes with you, then the title transfer and re-registration can be smooth and quick, too, but that’s not necessary.)
If you think he’s dodgy, then how certain are you that he paid you with non-counterfeit cash?
Next time you sell a car privately, read over the back of the title (at the very least) and the pretty clear instructions in the driver license manual, which you can obtain free from your DMV.
I would put in a call to your insurance company and explain what has happened. They’ll have good advice for what to do now. And you need to cancel your insurance on that vehicle in any case.