Over the years I've developed good relationships with several excellent repair shops and technicians, like Marflex, Frank Marshman (CameraWiz), S. K. Grimes, and Precision Camera Works, and as I see it, this is key to making good deals from private sales and eBay. I also have a good idea for most of the items I buy, what kinds of things are likely to go wrong, and what will be an expensive repair, and would would be an easy fix or possibly something I could repair myself.
I would rather buy something in non-working condition at very low cost from a private individual and have it brought up to spec by one of the best camera techs in the business, than to pay more for something in basic working order that may not have been serviced in a long time or not by the most qualified technician for that particular piece of equipment.
It is pointless, for instance, in my opinion, to pay more for a large format lens in a "working" Synchro-Compur shutter that hasn't had a CLA in 20 or more years, than to purchase the same lens in a non-working Synchro-Compur shutter that probably only needs a cleaning, because in fact, both shutters need a cleaning--it's just not as obvious with the "working" shutter. Sometimes a non-working shutter is really broken and may need to be replaced, but in most cases, they just need cleaning, lubrication, and adjustment.