What a lot of people failed to realise during the bubble is that the vast majority of this stuff is just not rare. Shimano produced thousands of groupsets at every level. There are plenty of M900 rear mechs on the front page of eBay now… you have to question why anyone would pay more than £30 for one.
I feel for anyone who bought into the hype and spent thousands on old bikes, but you bought during a bubble and now it has burst.
The sad thing is that a lot of people who overpaid are holding on to stuff in the hope that prices will go back up. They won’t, and a lot of kit will now rot in damp drawers.
Yes, exactly this. Most of the parts, and many of the bikes, were manufactured in their thousands if not tens or even hundreds of thousands - and 30 years isn't a sufficiently long time to make them rare. Finding pre-WW1, or even Victorian bikes, in great condition - now that's rare and in some cases, nearly everything on the bike was bespoke and handmade. Compared to some retro mountain bikes, they look much better value. There are exceptions e.g those with some history of racing and provenance, the one offs, the prototypes, the very limited numbers by boutique manufacturers etc.
Much of the high pricing on retro-mountain bikes was people just being misty-eyed about their youth when they couldn't afford the bikes in the mags - and being bored during covid.
The much needed correction is happening. Plus none of us are getting any younger and the passing years made it harder not easier to ride the steeds of our youth.
Regarding prices on eBay, the best option is to check sold prices and base any offers on those.