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You should consider signing the title. If you're worried about the chain of title being correct, you and the current buyer could just sign and date a statement between you that says that the you sold the car to whoever and that person sold it to whoever (the current owner). The reason I say you should consider signing the title is that if you're still the registered owner you still have legal responsibility for the car - and you don't want that burden if you don't own the car now.
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