MPB rates the camera in two things : functionality and cosmetic condition.
If the cosmetic condition gets down (some bumps, paint going away, earased buttons, scratched screens, etc) the price drops.
If the functionality of the item is lowered (dirty lenses, hard to turn focus / zoom ring, broken screen, non functional AF or image stabilization etc) then the price is lowered again, and the defect mentionned in the product description.
If the camera or lens has a functional problem that makes it unusable (broken shutter, camera not turning on, no screens working), then it's sold at a heavy discount as "spares and repairs".