If someone knowingly sells me a vehicle with illegal parts without disclosing said parts, everyone who will listen will hear about it from me - frequented forums included, but only after I have given them the opportunity to "make it right" and they have failed to do so.

As for the new owner's, this is a tough one. Although it is the competitors responsibility to know that what he has brought to the track is legal, it's unrealistic to expect him to do a full tear down on his newly acquired used race car.