This is not limited to cameras and photographic stuff. Lots of people with old books assume that just because they are old, they are valuable. They don't understand that their third printing of a common title makes it worth only slightly more than the value of the paper.
I see it a lot in antique tools as well, people equate their rusted hulk of an old Stanley plane with a pristine, in-its-original-box example seen in some catalog for hundreds of dollars, and figure they have a treasure.

Shows like "Antiques Roadshow" just make it worse.