I have no problems buying off Ebay - but there are some pretty obvious pointers to condition:
Out of focus photographs are hard to produce with modern digital cameras so if the photos ARE out of focus, the seller is hiding something.
Even someone completely ignorant of cameras can tell if knobs turn and buttons press - if they won't tell you so, either the knobs don't turn or the buttons don't press.
The converse is also true. They can't get a sharp image to save their life and the description isn't trying to deceive, it is just ignorance.