Same thing happened to me... Traded my car for another car, met up the dude at his work, switched cars, I went straight to DMV, he said he'd go on his lunch break.

Two weeks later I get a certified letter from a towing company saying they impounded the car, but were coming after me for it. They will suspend your license if the bill is not paid. Turns out the retard got caught street racing, with a suspended license, intoxicated, and with a ficticious tag on the car. He was arrested. Car sat in impound for a week, and cost $500 to get it out.

I will never sell/trade with out a BOS or meeting the person AT the DMV to personally see them transfer the car to them.