There's definitely an element of "wannabe speculators" in the old dirt bike game.
Seems to play out roughly like this:
1. Someone (usually a young bloke) finds an old bike in an uncle's shed, gets it for free/carton of beer.
2. They start researching, and discover that SOME old bikes are worth good money.
3. They assume their old bike is worth good money - because a KV125 is just like a 490 Maico...
4. They get into a circle-jerk with similar people, all talking up their TS185s, blown up XT550s, and 1985 XR250s fitted with DT125 motors...
The cycle only ends when the seller needs cash to register their shitbox VS Commodork and sells it for something like what it is actually worth, but not before berating all of the potential buyers for ignoring the "mad bargain" that's been "full rebuilt"...
Some never learn - they prefer to hold onto the dream of a $1200 bike being worth $5000, rather than actually taking the $1200.
So yeah, I pretty much agree with your assessment.
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A few years back, there was a noticeable difference in value between eras - Pre-75s and Evo bikes were worth more than anything else. Now, it seems that this has pretty much levelled out - there's no big difference between any of the eras.