So you really want the Nikon D810 and it would be the perfect choice however the price is not within the budget. Should you go searching for a used version of the D800 or do I settle for a cheaper new camera?
There is one rule that never should be avoided and especially in this case. TRUST! I have bought used equipment a couple of times and have the experience that 1 of 3 times it have been effortless.
First time was buying a demo D100 from a store and it really worked out well. The camera was hardly used and in mint condition.
Second time I ordered a tripod head online and the item itself was exactly as described and worked perfect. The only issue was that for some reason it was stuck in customs for a long while, when doing some research we found out it was stuck in Australia. I live in Norway so it took some effort to get it to my side of the globe.
Third time I bought a lens that was in very good condition regarding the optical part, however the autofocus motor was really worn. I did get a discount on the lens, and since I was used to manual focus it was possible to work with it. However it was a disadvantage.
So buying used dslr equipment may be worth the risk only if the one selling it is highly professional. A professional store would not put their reputation at risk by making a false estimate.
2. Be honest with yourself!!
If you do not wish to enter the fully manual world of photography, do not buy a to advanced camera just because it is cheap! If you go to buy a pair of shoe it would never be a good investment to get two sizes to small or big just because it was a really good offer. In reality the shoe will never be used.
If the camera do have features needed for your choice of use i.e weather proofing, fps (frames per second) or low light shooting then consider a more pro camera. A brand new D7200 will give great pictures and may be more suitable than a used D800 for about the same price.
3. Get pictures and data on the actual equipment and ask for more details than in the first ad. Ask also for the serial number since it do give you some information, i.e cameras sold in Europe have a beginning of 60, Asia 80 and so on. There are a few online sites that has this information. All newer Nikon also have a count on how many pictures it has taken (shutter count). The shutter on a camera do have an estimated lifespan and cameras used by a news reporter may be close to replacing the shutter.
Always but trust and need first. New items will always be safest however a safe second hand purchace will give a really good deal and save you some money. Would I buy second hand? Absolutely if my budget demands it, but I would save a little less and buy it used from a professional store/website rather than a private solution.