I would have thought Nikon would be willing to fix it no matter where you bought it. They will just charge you for it. I expect you could send them a 40 year old camera and it will get fixed for a fee.

Gray market or not, I've always been baffled by people buying "new" from eBay.... I have always thought of eBay as a used marketplace. No formal vendors, just folks selling stuff they got their hands on, maybe they got a truckload of something and perhaps they specialize but still just some individual selling stuff. I expect zero warranty unless specified and even then, how do you expect to contact them other than through eBay?