If the sale happened in the US, it doesn't matter where the product was produced, first sale applies. Apple can't in any form claim that an iPhone purchased in an Apple store in the US was, in fact, an unlicensed import.
Again, correct me if I'm wrong: But "first sale" is a long-lived and fairly unambiguous law. Unless it's found to be unconstitutional (which is not being questioned), the courts can't just change it.
The question before the court is what happens when the sale happened abroad, because US copyright law doesn't trivially apply abroad.
What gets murky, as far as I can tell, if first sale doesn't apply abroad are online merchants that can trivially fulfill your order from abroad, thus making you the importer - just like the Amazon UK case. Then you'd be forbidden from reselling that product in the US without the copyright holders permission.
That said, it's just about a guarantee that SCOTUS won't stomp all over the first-sale doctrine. It can easily keep both the first-sale-in-the-US rule alive as well as maintaining the rights of IP holders against unauthorized imports.
But... It won't necessarily be that narrow, and it is but one battle in a long war over who gets to control a good once it has been sold. It certainly would be possible for the Supreme Court to produce a ruling in this case that would see grandmothers being cuffed for illegal reselling at flea markets.