After selling a vehicle, if the new owner gets into an accident and has never re-registered the car, can the original owner be sued?

Question Details:

My wife, girlfriend at the time sold her car a few years back. Both seller and buyer signed the transfer of title and ownership. Apparently, the buyer never bothered to register the car under their name at the DMV. So they get into an accident with no insurance and no drivers license. Now the other party of the accident is trying to sue us, and hold us liable for this accident which took place years ago for which we did not know about. How is this even lawful and why does their lawyer calling us harassing us about this? Shouldn't they pursue the individual involved in the accident?

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