IMO, best practice: anything that was originally included by manufacturer and is not present at resale should be explicitly mentioned.
That said, the grills were not pictured, and seller did not actively mislead, so he's got a case; I doubt what happened rises to the level of fraudulent misrepresentation.
(A friend of mine one sold a very old BMW 5 series, and the frame broke the next day. In small claims court, the judge ruled that to get relief, the buyer would have to show that he was *prevented* from inspecting the frame.)
Perhaps a fair solution would be for seller to offer some consideration towards new grills (50%?), so everybody could walk away feeling reasonably whole.
I'm optimistic about such a concession, though, esp. since the cost of replacement grills might be extortionate,