Used is a risk with a camera that is so expensive to repair and most defects occur when new or at end of life, unless damaged by the user. In this latter case,warranty is not invoked. Check with the retailer for the grey market version as to who does their warranty service. It makes a difference if they offer a long warranty and are competent, it is worth extra.
The D800, and D810 are well sorted out so either late version D800 or new discounted D810 is a worthy camera as you know from having the D800. I shoot with friends who have D810s and for all intents and purposes, they are essentially the same high quality images.
The latest rumor, from a Chinese Nikon ad, is the D850 will be announced on August 16 and surely D810s will not be built in tandem on the same production line, Nikon does not have spare capacity. If it is intended for fall lease to stores, they are in full production now which means no more new D810s have been made for a while while they changed over production. There probably are no large stocks of new A stock D810s left but surely there will be Refurbished available for some time. Personally, I prefer the refurbs over the Grey market, because they retain factory warranty and are offered at better prices than A stock grey market.
Here, out of warranty repairs are so reasonable, a good used one makes sense for limited budgets, but in Europe or the US, repair is very expensive.