physical defects that have been mentioned, have not been mentioned, whether you accept them
corrosion on the metallic ends, i.e. battery connections, plugs
shutter count so that you know how far (roughly, no one can give you exact shutter life) you are to a replacement being due
but frankly speaking, there are some things that you cannot tell.. so if you do not want to take the risk, just buy first hand. not all sellers are reasonable and some even seek to deceive. caveat emptor.