Admittedly i don't really know how the law would operate under this specific scenario either, but i would like to think that if an auction house presents an item like this in a cautious manner (i.e. this item is made of some "yellow metal" that has been fire gilded) but last minute puts out a public notice that they just discovered the item in question is really hallmarked gold, thereby raising the perceived value of that item in time to affect bidding, but that once the item is received the winning bid discovers that said item is, in fact, not solid gold, that a court would consider that a case of fraud. But again, that would be my hope. I have no idea how the law works.