Here is a checklist for buying a used camera. (Note: some of the info here was derived from a post on nikonians.org on this subject.)
When was the camera purchased by the seller?
Was the item originally purchased new, used, or refurbished?
Local version or gray market? If gray market, manufacturer might not service.
Reason for selling?
Ask about seller's photography background? What kind of photos does he or she take (portraits, landscape, still life, wildlife, etc.)? (The more knowledgeable the seller is, the more likely the camera is legit as opposed to stolen.)
If the seller is pro, ask to see their portfolio. In my view, buying from a reputable pro is safer.
i. streaking (damage due to improper sensor cleaning),
ii. against dark background, check for hot pixels (usually can be mapped out relatively easily), and
iii. dust on the sensor (normal and easy to remove unless it is stuck on the sensor). check for dust on sensor by taking photo of clear background, focus on infinity and use the smallest aperture (f/22 or smaller).
i. 2xxxx japan
ii. 3xxxx USA
iii. 4xxxx Europe
iv. 5xxxx Canada
v. 6xxxx Asia/australia
vi. 8xxxx UK