Welcome to Nikonians! While Nikon Service will not work on Gray Market gear all of the Nikon Authorized Service Stations will service Gray Market Gear.
When buying used gear, It is best to buy from someone that you trust. KEH prices are at the top of the market but they stand behind everything that they sell by providing a 6 month warranty on the used gear. They also rate condition conservatively. Good Luck and Enjoy your Nikons!