At least in the US, the law is pretty clear on title--especially dashing any defense of 'innocent purchase' (bona fide purchase)...The car was stolen, it still belongs to whoever it was stolen from. And basically doing anything with the car is a potential for being accused of handling stolen property.
In this case, very possibly the seller was an innocent purchaser--that really gave up cash to buy the car at a reasonable price and had no way of understanding otherwise (lets pretend, bought it as a barn find, or out of a storage unit for $15k...maybe even damaged, whatever). Except for not looking themselves for a Vin, lets say their judgment was impeccable and the deal didn't clearly stink. But then, sorry, in US law this buyer is now the second victim in the chain--being a bona fide purchaser doesn't protect stolen property.
Legally, whoever has the car right now doesn't have to a thing...even, doing anything other than calling the cops may be illegal. Whoever is in possession of the car doesn't need to call the cops, but if they 'hide' it or sell it in pieces (much less, imagine selling the whole car) they become the Krimi Verbrecher themselves.
So recourse is naturally to get their money back from the seller--which really, should be assumed as a starting point...The good news, some justice could be done to the original owner or maybe their estate...Lee