I know it can happen. Your sining a legal government documents when you check the box for at or below fair market value. So you better be able to back it up when they call your bluff!
but then again, a sales recipt private party to private part saying “I am selling my RX7 VIN XXXXXXXXXXX for $1000 as is (blow up motor) to so and so” I would assume would cover that bluff if they asked why it was so cheap
Bills Of Sale also need to be notarized to mean anything.
Think its best to gift him the rx7 and he gifts you the m3
Sign both cars over to me and I’ll take care of the regi 
don’t you have to pay tax on a gift unless it’s family?
i sold my truck to a guy in vt, he was telling me they have to pay tax on nada value, no exceptions, that really sucks
CT is also nada value. Sucks! As for this case, just write it as a gift from “cousin”. Done deal.