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.

good advice. your town will still charge you excise tax as well if you don't
cancel the plates, not to mention possible liability exposure. luckily in Mass. you can cancel online in 2 seconds practically.
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'91 244 205k auto