For the price of a normal on-the-market or on-the-auction used D700, you can probably pick up a refurbished one with a 90 day Nikon warranty. I bought a "normal" one used one from an individual on Amazon (1,100 clicks) and a year later bought another--a refurb from B&H (3,500 clicks). B&H doesn't seem to have too many refurbs (though I did just pick up a refurb 70-300VR that was "hidden" in the used section that was cheaper than a "normal" used one sold for at KEH). Adorama and Cameta seem to stock a number of various Nikon refurbs on a regular basis. As far as what to look for--all of the usual suspects--signs of heavy wear on the body, all functions like modes and autofocus running correctly (don't forget the pop-up flash), and shot count averaged with the reported age of the body. The D700 is a fairly robust body. It should not show too much abuse. If it does, I would pass.
Not many D700s seem to show up on the used market, and when they do, they don't seem to go at much of a discount. I agree that getting a Nikon refurbished model may be a good alternative at not much more cost than a uased model of unknown lineage. I'd also shop around with the known reputable sellers and watch for those little price "down burps" that show up occasionally. But on the whole, I think the low number of used D700s is pretty solid testimony to the camera. When a follow-on model appears, those who simply have to have the latest model hanging from their necks as opposed to really taking pictures with them, may let a number of them go, but that's conjecture. And you will still be sans a warranty. I shy from the extended warrantees, because electronics either seem to develop problems early on or never. Again, that's my opinion and I may be burned someday. I do use a platinum card to order them and that doubles the warranty for free, normally.
Joe -- Concerning doubling warrantees with platinum cards, call your credit card company for details. I have two Master Cards and a Visa from three different institutions and both provide this benefit. I have to keep my invoice and if the machines die after the first year, then the card carrier pays for repairs. This provides an added year on a one year warrantee -- it does NOT change a 5 year lens warrantee to 10. In fact, on the longer lens warrantees (US, not grey market), I think there is no benefit. But for 90-day or one-year warrantees, it works. Understand, I have NEVER had to use this benefit, so I cannot comment specifically on how they provide this, but I have no reason to doubt it and I have confirmed this every time I have bought a body -- my D200 and D700.
I have owned a refurbished D700 for a year and have had no problems. I was told that Nikon has such good quality standards that it is cheaper to replace any cameras with new ones and recondition the problem cameras. A refurbished is sure to be in good working order.