1) You have to be reasonable as a human being.
2) If you feel like you have the right to look over something before buying, whether a car, walking through a new house, clothes, knocking on that melon, looking over a bunch of bananas for spots, or searching for a camera, then you have to expect that when you buy one of these, somebody else thinks like you and did the same thing.
3) It is one thing to be a conscientious consumer and find discounts reasonably (price matching, coupons, etc), but to use the fact that somebody might have touched an item, regardless of the size or depth of a finger print, etc to get a discount seems a bit too unreasonable, and it is purely selfish rationalization to argue otherwise. An exception to this could be that a fingerprint is on the mirror or sensor of the camera, or the fingerprint is on the glass housing of a halogen light bulb, etc. There are some parts of electronic equipment that are not to be touched.
4) A vast majority of items you buy "new" have already been through several hands, at some point in the manufacturing, distribution, and sales stages.
If an item is damaged, that is a different story, and has a different answer, and things like a fingerprint on the mirror, sensor, halogen bulb, all constitute "damage" at that point.