Mudguard,
I agree older bikes are perfectly suitable. I have a 2009 model that I absolutely love and I won't get rid of it, but it came from someone that only put about 20 miles on the frame by the time I bought it three years ago, and it was set up as a single speed with bmx bars and every part on it was junk except for the brakes. I put good stuff on it and it has been my favorite ride for years.
If it had signs of hard use when I looked at it, I might have passed it up, because the risk of problems would be high enough that I could spend my money more responsibly on something else.
I listened to a pinkbike interview of Gary Fisher a few days ago. He said one of his riding buddies when he was racing as a teenager would only buy Schwinn bicycles because they warrantied the frame. He was such a hard, powerful rider that he broke a frame every year. He said everyone believes that a steel frame is indestructible, but if you ride long enough and hard enough, you can destroy them.
I know a steel frame can be repaired, but that's a big hassle I don't want to have to go through on anything I buy, at least not until I've put it through its paces for many years myself. I don't want to buy something that has been ridden hard and put up wet only to have to try and repair it within the year. I may be wrong, but I don't think you can repair an aluminum frame. I've heard people say that you can, so it might be possible to do so, but I think you have to have incredibly specific equipment do it, and it's out of the realm of possibility for most consumers. Most bicycles that are older have aluminum frames and have seen a ton of use. Not only do they suffer a lot of cosmetic damage that simply makes a bike look bad, it suggests they have seen so much use that the frame might be nearing the end of its life. The components on these older bikes have seen years of use too. They may need to be replaced soon.
All of these things represent a hassle and an extra expense to figure into your purchase of an older used bike. This should bring the price down compared to newer used bikes that don't face such a high risk of having to deal with these problems. Sellers, and a lot of the people in this thread, are ignoring those facts.
Component functionality has changed over the years too. I had a bicycle with an XT groupset from three generations ago. My hardtail has a Deore groupset on it from two generations ago, and it performs better than the older XT stuff I had. Components from 10-20 years ago don't have the benefit of all the technological advances and don't have the trickle-down technology the newer parts have, so this also represents a value to consider in the pricing and purchase of a used bicycle. Yes, you can buy new things to place on these old bicycles, but then you're just looking at more expense after your purchase, which a buyer would need to figure into his total expense. So a seller needs to consider that the componentry may be substandard compared to what could be bought on newer used bicycles and adjust his price accordingly. If a bike is being sold with wear items that will soon need to be replaced, a seller should consider this in his pricing as well.
It just seems, historically even before this pandemic, that sellers ignore these considerations when pricing an older, used bicycle. Maybe they're hoping for someone who doesn't know much to think the deal they're offering is a good one, but for someone who is informed, we're probably not going to pay the lofty asking prices.