"...I should have checked closer." Chill Dave, please no need for paranoia... your D610 will likely out live your need for it... a decade ago I purchased a "UK Gray Market" D7000 as a backup for the one I was currently using... The D7000 was released in September 2010... That "UK" D7000 is still currently in use by one of my second shooters... And still going strong...

btw, KEH will likely service your D610 as will many other independent shops... Yes there may be an issue with parts but KEH likely has as treasure trove of the aforementioned... lol

Best Advice? Over a decade ago my primary revenue stream came from assisting other commercial shooters in my market... Most told me that they never purchase their cameras new! Always "Refurbished" from their camera's vendor. Reason, new cameras are only "spot" checked on the assembly line... while Refurbs have been carefully brought up to "spec" by skilled crafts persons... Nearly all my cameras are refurbs now and they have been stellar!

Dave I'd worry more about your D610's challenged AF focus system... I was not impressed with this kluged implementation of the D7000's 39 point system in the D610. It preformed rather poorly for shooting sports/action... I sold it after struggling one season with it... Great IQ, albeit challenged in low light AF performance... My work around was to use only the Center Cross Type focus points... Yes the D610 is great for landscapes and portraiture (with adequate light)

Hope this helps...
All the best with your Korean Nikon, Thinking your clients may never ask you if it has a US warranty...

If it plainly said it was a Korean sales model than you have no argument. But if it didn't say it was non-Nikon USA etc on the ebay site call them and see if they will do anything. Sometimes they will, sometimes they won't.

In future check things out completely and if you have questions don't wait a month.

Also if you use "quote reply" we know exactly what you are replying to.

I shoot Canon but from my friends who have Nikon they have the same experiences I do, no repairs except for damage (I had a tripod go over once).
Keep the camera in good shape, have a local shop clean and adjust once in a while, be careful to avoid chances for damage and update firmware when they bring out new versions. It should give you a long time of good service. If it should need repairs the model is old enough an independent shop may have a damaged camera to get the needed part from. Well and there are a few large independent shops that import parts from other countries - it can get expensive but they can get parts that way.