I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today. I have been a licensed attorney for more than 3 decades.
The pure and simple legal fact is that you are not responsible for the car that was sold more than 2 years ago, left languishing in your yard all that time because the buyer never picked it up and then was stolen.
The buyer (true owner of the shell of a car may legally make an insurance claim for the theft if he bothered to insure it, but absent that he is out of luck. You DO NOT owe this man any money back because the car was stolen through no fault of your own. His lawsuit is ridiculous and you will not have any problem winning and lawsuit that he might file against you.
You are not responsible if the car, which was essentially abandoned by this idiot, was stolen 2 years after he bought it and promised to remove it. The court will rule against him in his lawsuit.
You may reply back to me using the Reply link and I will be happy to continue to assist you until I am able to address your concerns, to your satisfaction.
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I wish you and yours the best in 2016,