If I sell a car private-party, all that I have to do is to send to the motor vehicle department a notice stating that the car has been sold. It doesn't necessarily mean that the title is transferred, but it does notify MVD that I have upheld my end and told them that I don't have the car anymore. It's the buyer's responsibility to register the vehicle. Admittedly, with visible license plates (which the seller is supposed to pull when the transaction is completed) it should be hard for unregistered vehicles to drive around, but it does still happen, either with expired plates/tags or with a stolen front-plate from the same model being applied to the rear of the car.
Similarly, since there's no required registration for firearms, if I sell a firearm that I had purchased new and therefore was registered to me, if the police later come to me I would probably not have to do a lot more than tell them that I sold it for cash. Unfortunately for the investigation that's probably where it would stall. I might find myself under law enforcement scrutiny myself for a bit, but so long as my explanation wasn't contradicted I'd probably be fine.