Wow what a mess. You should not be out the truck and car but you need a litigator to write a letter to the jerk who is trying to have everything both ways.
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You need to consult with a litigation attorney a who is familiar with auto title issues.. Unfortunately, if the seller is unwilling to comply with the terms of your deal, you will most certainly be forced to go to court to try and straighten this matter out. Do not delay in acting. Time will only make this case more difficult to prove.