How can I trade cars with someone and avoid sales tax?

How can I trade cars with someone and avoid sales tax?
My mother in law has a Honda Accord and I have a BMW. I would like to give her the BMW and take the Accord. The problem is that in order to register the cars we would need to pay sales tax on the MARKET VALUE of each car. Is there a way we can trade cars and avoid paying sales tax? My car is registered in Connecticut. My mother in law's car is registered in Mass.
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