I can see the sellers perspective in those situations. They want to clear out the shed, but selling something for less than they value it is daft, and they might as well keep it.
It's materialism - things have a value to the owner, generally monetary.
I have a set of hope/mavic 231 wheels. They've have been pristine since the day I bought them (1994). I'd happily sell them for £200, but I really doubt anyone sees them worth that, but equally, I wouldn't let them go for less, as they have a value to me. I'm sentimental.
And in equal measure, some people are just greedy, and want to see as much as they can get for bits.
I guess the niche heyday of retrobiking has passed, and more money is circulating within the Retrobike world as more people rediscover their old bikes.
To Me, it's a remarkably cheap hobby, even though my trek build has seen around £1k, and isn't finished yet. I have a job that pays well, and no kids. my other hobbies cost a LOT more than retrobikes, so I tend not to get worked up about prices - parts are worth the asking price, or they're not.