I don’t understand why this is so hard. If you look at enough cars, you can get a feel for what different cars are worth. And it’s pretty obvious that the prices listed for online sales are high for various different reasons. First, that’s the sellers price. Second, most of those services charge a fee. And they reach a large audience, so sometimes a car will get bid up disproportionately because it just happens to have a couple rich guys looking at it. But again, if you look at enough cars those kind of things stand out.
Comparing two cars straight across is pretty difficult too if you’re talking about classics, especially if you’re talking about classics that are 50+ years old. The odds of seeing two different cars that are identical in condition and overall value (think options etc) is pretty small.
And yeah, what something is worth to one person may be worth something different to someone else. I’d never pay extra for a car with brand new rebuilt suspension is it was all OE. I’m gonna toss it in the recycling bin, so whether it’s all brand new Moog garbage or 50 year old factory stuff makes no difference to me. That’s an low percentage example, but people will favor certain things based on their own skill sets.
Regardless, there’s certainly no duty for anyone to post what they paid or what they sold for. If they want to, fine. If not, that’s their business. And for everyone looking, how the heck do you know some random guy on the internet isn’t exaggerating how great a deal he got. Or bumping the price for a future sale? There’s almost no way to verify that.