I was going through some receipts the other day and saw what I paid for my Nikon 880 p&s digital. I still use it (fine for documenting items for insurance or sale) but I'd only get 10% of what I paid if I was pointing a gun at the buyer.
Virtually all of my current film equipment (35mm, MF, LF) was bought used and carefully. So pretty much whatever depreciation I've encountered has been more than compensated by use of the equipment.
Craigslist (at least here) tends to swing both ways (i.e. a Minolta SRT101 with an asking price of $295 with a 50mm and a 3rd party zoom to a SRT202 with 50mm and third party zoom for $29). That's OK. The guy/gal asking $295 is ignorant of camera values; I'm probably ignorant about something they know a lot about. As long as it's fairly represented I don't mind. The market will tell them they're wrong.
But some sellers can be ridiculous. I called about a Nikkormat that was described as "works great". When I asked, I was told the meter doesn't work.