After living thru a cautionary tale on buying used, I would add one thing to this list.
Check the accuracy of the multi-function toggle. If you land on any of the wrong menus-IE-line above or below/n/y instead of y/n, make sure you have it checked before buying. D300 I considered buying had a loose toggle.
Loose with damaged electronics underneath it as it turns out. Nikon charges a "standard fee" of $400 just to open up the camera. If pricey parts are involved, the costs skyrocket.