Is there a difference or not? From what I've read, gray vs US market items are produced side by side.
MAYBE a lens sold in the US will have the letters US preceding the serial number. Not all US lenses do. I suppose this is because they produce x number of lenses they know are headed to the US, and the remaining ones probably won't be. But circumstances may dictate they take some of those 'non-US' lenses and package them up (complete with US warranty papers) to ship to US distributors.
But take a lens sold in the US and set it beside a lens sold in England, or France, or Brazil........... the letters US on the serial number MIGHT be the only distinguishing item (IF the letters are there at all).
I'm sure Nikon keeps track of the markets certain serial numbers are sent to. So if John Doe buys a lens in South Africa, then sells it to someone in Kansas, the poor slob in Kansas may send in his used lens to be repaired and get told one of two things: It's a gray-market lens and no warranty work will be done (and if out of warranty, they still will refuse to work on it no matter what, even if he offers to pay for the repair), or there will be an additional fee to repair the lens even if it's under warranty.
Given that Nikon's customer service sucks, and I've heard many stories about someone wondering if their lenses might be gray market because they bought them used (like the guy in Kansas) so they call up Nikon to find out only to hang up with no answers, you're kinda on your own when buying used glass. Heck, even my local brick-n-mortar store (which is an authorized Nikon dealer) can't call up Nikon and find out if a lens they have in their used gear display is a US model or not.
When buying used, you pays your money and you takes your chances.