If you sell a car, turn in the plates immediately.

At the fire station where I worked, we let some of the neighbors park overnight. One time I noticed that an older station wagon did not move for several days. Upon investigation, I found that it was parked a bit into the bushes, had no rear plate (at the time Mass, did not require a front plate), and was full of junk like old tires, batteries and trash.

The police found that it was still registered to an old lady in Boston - she had given the car to a nice young feller - she had to get it towed away and disposed of. I thought she was sick over the matter.