One thing that I have commonly encountered are cameras with shutter problems. Shutters that don’t work at all, ones with incorrect speeds, or ones that don’t have working slow speeds.
Many older cameras had Copal or similar type shutters.
Restoring these to working order is not that difficult … though getting into them may be difficult in some cases.
The lower section controls the slow shutter speeds, usually less than 1/30x. These gears get gummed up. The easiest way to clean them is to use Rosonol. Apply the lighter fluid and then work the slow speeds. You probably will have to do this numerous times … and then you might have to do it again after the Rosonol evaporates.