You've sold the car and the cheque has cleared, but the job is not quite done. Now you'll need to complete the 'notice of disposal' form, typically found on the back of your annual registration form.
If you can't find that then your state roads authority will have forms available. You need to sign it, as does the new owner, and then each of you must take a copy into the registry.
What happens if you don't? As far as the authorities are concerned the car is technically still owned by you, and therefore you are liable for all crimes committed with that car, from simple parking tickets to hit and runs, bank robberies and more.
A few minutes of your time in a registry queue could save you years of time behind bars. Get state specific information at the following places: