Not everything is a winner, Wiltw - Some items lose value or simply depreciate with age or wear, while others take on a cult lens or cult camera status, worthy or not. I just noticed that my model of 8x10 camera, purchased about 40 yrs ago, is selling (or trying to sell for) twenty times as much as what I paid for it. That's on average! Most of my 8x10 lenses have quadrupled in asking prices from just a few years ago. When it comes to MF, my beloved 6X9 rangefinder cameras have doubled in price in just the last two years from comparable condition ones, and it certainly isn't due to any lack of scarcity of those models out there.
Then you have exchange rate games. There was a year when the Japanese Yen was at a low against the dollar, but the German Mark quite high. So I sold off all my heavily used Schneider lenses and made enough profit to replace them with brand new Fujinon lenses of even later engineering.
It's all in the timing. But for me now, I'll refrain from selling anything as long as I need it or can realistically still use it. If it's redundant to me, or when I simply get too old to lug it around anymore, then it's fair game.
The local market is interesting too. The main camera store nearby would accept trade-in vintage 35mm and MF cameras, and they'd
almost immediately sell, after inspection and cleaning, especially anything classic mechanical Nikon or Canon. When that same store was massively burglarized one night, the used camera cabinet was totally emptied, and only afterwards, apparently, new digital stock partially taken. Either the burglars had their own photographic preferences, or more likely, knew that it's easier to sell off something older at a flea market somewhere than what looks just too obviously heisted. But if the demand wasn't present, they wouldn't have bothered.
They knew what they were doing ... the cops showed up 3 hours later than the alarm went off, and slightly after the thieves themselves were gone. Convenient, especially for a city which boasts in a 3 minute emergency response time. But that's another story.