if you want to get your camera from 2nd hand market, it is risky,as you do not know how the previous owner handle it, how much shutter he taken, any hidden problem, he drop the camera before or not (he may tell you no no no, I handle it with care, but who know?)

need to check it is still under warranty?
how old is the camera (when he purchase)
all buttons working
all mode dial working, no clocking when turning
top LCD panel and back LCD display working properly
the lens can mount and unmount smoothly
and those external connection like USB, shutter release, you might not able to check on the spot, and depend on your luck if it working properly.
display in viewfinder working properly
focusing working properly (check focusing indicator light in viewfinder also)
test take picture, shutter move smoothly (this one difficult to tell, but if you experience, u will feel the shutter movement sound not normal)

last but not least, check the price, it the price is lower than 2nd market normal price, something might went wrong. the camera may be a stolen set. there have been case where a user get a 2nd hand camera, when send to service center, that found from the serial number, the camera was reported stolen. that has cause losses on the 2nd hand buyer.