When you're on either end of a private vehicle sale, you need to keep a close eye on all the paperwork, otherwise the consequences can bite you years down the road. Mercedes Streeter has sold dozens of vehicles over the years, but this one time she let a buyer drive away on a motorcycle with her license plate turned out to be a mistake. Four years later, she started getting tickets in the mail from the city of Chicago. A little digging revealed that the motorcycle had changed ownership four times, and none of the buyers bothered to title or register the bike. Streeter had sent in a form notifying the state of the sale, but it was apparently never properly filed. She will be going to court to prove the sale four years ago, because she doesn't trust the process for dealing with the state online or by phone.
The laws on selling a vehicle, or proving you do not own it anymore, vary from state to state, as explained in the comments full of people with similar stories. Around thirty years ago, I sold a Supra privately and received a property tax bill a year later. My only recourse was to pay the tax and then declare the car completely junked. Since then I refuse to sell a vehicle without meeting the buyer at the DMV for proper registration. Before selling any vehicle, you need to have the paperwork completed for your state to absolve yourself of responsibility going forward. Read Streeter's story at the Autopian and take heed from now on.