And you tell them to piss off like most shops would (ever seen pawn stars where they talk people down quite a bit or show them the door?).
That's what a good shop would do. But then that's the reason I think it's more a fantasy than an actual reality, moreso due to COVID-19.
For starters, you have the problem of idiots. Some guy walks in with a 12 year old Core 2 Duo and is upset you won't give him $900 for it because in his eyes it's "vintage" because it won't run Windows 11. Then you have the guy with his generic 486 AT Clone who thinks it's worth $25K because HE has never seen one before. Then there's the guy that tries to sue you six months later because he finds out he could have made $2500 on his IBM PC 5150 on E-bay.
Some other thoughts this raises....and this might be a mouthful.....
I know we consider this niche but it really is not so much anymore, and has not been since maybe, 2010 or so. If you think about it, we have a nice lot of heavy-hitter vintage PC YouTube Channels (8-bit guy, Adrian's Digital Basement, LGR, PhilsComputerLab, RetroSpector.....just to name a few), places like RE-PC are starting to offer parts for these older systems in their stores again, I bought 2 motherboards in Tukwila before I moved. The owner of Computer Reset hits his deathbed and now there's a Facebook group busting at the seams with people wanting to get inside and buy up all the old PC stuff there, probably more than any other genre mentioned. Heck, I go to work now and I end up meeting other people into this now, instead of being the "Weird guy with the old PC's that are dwarfed by the power of my smartphone!" like it was when I was doing this in the 2000s. It's almost darn near mainstream now. My last 3 weeks of work I've been talking DOS games, x86 PC's Pre-Pentium 4, and so on with no less than 4 different people I work with, and got a SS7 and 486 from one of them. Even this thread itself is proof of that, someone wants to see pictures of a massive warehouse full of old computer stuff.
As such, I think the problem we have is determining the true, actual, market value of this stuff to keep people in-line with the prices yet keep people from getting ripped off. I get that a lot of us started doing this because it was cheap, but those days are gone now save for the rare case here or there. It is somewhat in the mainstream consciousness. I think some people are in denial of this because it upsets them. There was a time when this stuff was worth nothing and you could back your truck up and buy out all the gear and have endless amounts of fun with it, but those days are long since over and will never come back.
Now, there is a light in the dark of pricing. Eventually, older platforms will come down and level out as their respective nostalgic patrons start to get old and pass away. It's already happening with the Atari 2600 and some of the older consoles from that generation back because the young kids, the only thing they care about is the Nintendo. I can also see it staging with XT's and AT's as most of the younger folks into this, like one of my co-workers, tend to want something 80486 at the oldest. As folks my age get older, prices of that stuff will go down, and you no longer will be looking at $300 copies of The Secret of Monkey Island.