My experience was I paid very fair prices for a user Leica and lens, from different dealers around 1989/90, excellent camera & lens a few very minor marks and tiny ding but mechanically & optically perfect. Then late 90's the value dropped by around 50%, I could pick up similar and there were plenty around and prices stayed low for some time but I guess the pound was rising, it went from near parity £1=$1 to close to £1=$2.
Leitz also controlled the second hand pricing and quality through their dealers by keeping new prices high and not flooding the market with new products. A consequence was that Leitz dealers were doing very well selling second hand Laica's & lenses plus a few new ones, and actually providing customers with a level of service quite unique in the photographic field.
As an example my M3 hit a tree
as they do
Well I guess while wading through a thick wooded area it hit a branch, the view finder glass cracked, no other damage at all, it was covered by insurance, so I took it to the dealer, Leitz repair said £120 because we won't let a camera out of the workshop without a full service, when I picked it up the charge was half the quote, they knew the camera had n ot long been serviced & exactly who by as soon as they opened it up.
That impressed me, not because they'd read the pencilled service code inside the body, but because despite being expensive they were fair and honest. This seems to be the case for all the Letz dealers I've used or visited in the UK.
Ian