Normally people are very apprehensive about buying second-hand cameras with a lot of shots, but if it is from a private individual, and you can see it and try it before, I think it is just as safe as a store since with second-hand objects stores tend to escape/evade their limits "Responsibilities" if there is a problem after a few months.
Any models like D3 supports a lot of use and shots, you need to inform before about the camera, 200k shots in other models could be a problem.
WWW like this helps :
nikon d3 | Camera Shutter Life Database
Nikon D3 Shutter Life
Very Important what do the last owner with the camera, in the shop you don't know this data, only you know the number of shots and the price.
Only you know that the shop "test it" usually very fast check, before buy it and rated by appearance, usually the external appearance rules for the items at shops.
In private sales, you can meet the previous owner and have a feeling about the seller and the story about the item and check and test more than one shop.
In my experience, the shops are usefully to buy brand-new cameras or lens, but second hand the risk sometimes is higher, and you need to read carefully what kind of warranty have for second hand items, or you will pay more for an item with the same warranties that a private sale "NONE".
For the moment, the camera D3 is working fine as expected, I will use it as main camera during September to check it well, later would be my backup camera.