Speaker purchase question

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,

That's too bad, mcp.  Given that there are some shades of grey here, I'd probably not want to get into a hassle about negative feedback, even though the seller did not particularly distinguish themselves.
I personally prefer no grills (though a rowdy Leonberger pup forces them on me at the moment), but mijostyn's suggestion is spot on, if you like them: a manageable DIY project.

One thing that can be helpful: arrange a phone call with seller before any significant purchase.  One still has to ask the right questions, but details are more likely to come out even if one doesn't.  Also good to get a feel for things, before send a chunk of change into the void.