Well...without more information I could only speculate, but I don't think this car has "slipped through" anything.
Assuming it wasn't done via a Registered Importer, the car could have been brought in legally one of two ways: Either it was imported by a diplomat (or someone else attached to a foreign embassy in the U.S.), or it could have been imported by a member of the armed forces of another country on assignment in the U.S. In either case, at the time of import a DOT form HS-7 should have been completed (box 6 or 12 checked, depending), and also EPA form 3520 (code N checked). Those forms require the importer to declare that they will not sell the car while it is in the U.S. and that the vehicle will be exported when the importer (e.g. the diplomat) leaves the U.S. Making a false declaration on the DOT form carries a fine of $10,000 max and up to 5 years in prison. Making a false declaration on the EPA form carries a fine of up to $320,000 and five years in prison. None of those penalties would be of much concern to a diplomat, who would probably have immunity from U.S. laws anyway, but having been imported illegally, the car could be subject to seizure by the U.S. government, regardless of who owns it or how many buyer/sellers there may have been since it was imported. As the buyer, you wouldn't be subject to the penalties levied by DOT or EPA (since you didn't make the false declaration), but you might have to forfeit the car on the chance that federal agents come looking for it when the original importer leaves the country without it. Only you can decide whether that's a reasonable risk to take.
That said, getting a title for an illegally imported car isn't difficult in many states, but having a state title/registration/plates does not necessarily mean that the car is in the U.S. legally. A Virginia title would be issued by the state of Virginia, not the U.S. government. Totally separate entities. It's completely possible to be in compliance with the laws of the state of Virginia and still be in violation of Federal law (i.e. the laws of the United States).
Aside from the previously mentioned possibility of the car having been imported through a Registered Importer (which doesn't seem to be the case), I don't see any way it would be completely legal for this car to be sold here. I think you'd be kidding yourself to believe otherwise, but if the seller can provide an explanation that verifies this car's legal status in the U.S., I'd be interested to hear it.
