There is nothing wrong with getting a refurbished camera (or lens) provided:
1. Get it from a reputable, reliable retailer or the manufacturer. 2. Make sure it is factory refurbished, meaning by the manufacturer. 3. Make sure there is a warranty that is a reasonable length of time. 90 days is typical. 4. Make sure you can return it if not satisfied, 14 day minimum, 30 days is better. 5. Once you get the camera or lens, check it out thoroughly to be sure it functions as it should, and that it is something you that you can work with. If not, return it for an exchange or refund.
In this regard, the D800 is no different than any other camera or lens. There are known issues with some copies, which means you would know what to look for to determine if it is functioning correctly. Take those simple steps and there is no real risk to you except some inconvenience.
I have bought several refurbished Nikon bodies, lenses and speedlights, as well as 3rd party lenses. I only have had one bad experience: a lens that would not AF, bought from a manufacturer's on-line store. I returned it withing a week, and they repaired it and had it back to me in perfect working order a week later. A minor inconvenience but a significant savings over a the price of a new lens.