I don’t know if this helps, but this is a thin market. While lots of people may buy something occasionally, I don’t think you have anything close to the number of regular buyers of, say, old comics, let alone old cars. I also suspect the number of large ticket regular purchasers is less than 1,000 at any given point in time, not counting dealers. I am also not counting purchasers like me who swim in the low end of the market; there are probably a lot of us. So, wild swings should be expected on high end items, particularly if you start segregating the large purchasers into subgroups like Kirby fans, EC, etc. To me, an expensive Kirby piece is worth nothing because I would never buy it.
The impact of a thin market would explain what you saw. Sometimes, an expensive piece sits for years—yet the price goes up. I saw an Adams cover which the dealer wanted $13,000 for (no, not Coollines). When he had it for sale two years earlier, it didn’t sell for $8,000; and he would have taken less. And that isn’t even too expensive.
The bottom line is that this stuff is not a good investment vehicle unless you get something the market treats as hot, or comes with special background, like early Ditko. And that’s why there is no true value to most comic art.