I think sellers don't know what to charge because there really is no standard.
I'm into at least four different "hobbies": tech, cars, guitars, and classic gaming....out of all of those vintage computing is the most lacking in standards or decisions on what things are worth. This has been a boon and a bane because on one hand, cheap deals are possible, but on the other hand, some people have an overinflated sense of worth about this stuff.
In guitars we have a blue book that's updated to fit the current market.
Same with vintage cars.
Vintage Games and Consoles have had rarity lists and price guidelines as early as 1996....yes I've been "in it" that long.
But computers it seems have a very fragmented pricing base due to issues dating back to when they were new.
A fine example....my experiences recently at a local computer recycler regarding Model M keyboards......
I come in, there's a guy aware of retro computing, asks $40.00 for a blue label Lexmark made IBM Model M with removable keycaps. I'm ok with the price, that's still cheaper than eBay, but the shop already closed, so he put it on hold.
I come back 2 weeks later when I finally have time and grab the same Model M and a serial cable. - $4.00 for everything, different clerk. The clerk "let me have" the Model "m" for "free" because they are "not allowed to sell oooold PS/2 stuff". Because it's "Obsolete".
What I think we're lacking here is a standard or guidelines that are solid enough to say xx is worth this much and yy is worth this much and actually be able to justify the pricing. That and a lack of awareness of what we are doing, how, and why.
To the layperson we seem like a bunch of bottom dwellers who can't afford new computers, because all they know is BlueKeep, Wannacry, old computers are baaaad because they host viruses like that (even though none of them work on windows pre-XP) and the new iPhone 12 and how much bragging they can do when they get one.
Next are people who are tech savvy but don't get it because they are lazy. To them, anything requiring a keyboard is "too much work" and "cheaper/more efficient to upgrade to a newer model". Basically, new school IT people who don't know a memory address or IRQ from a hole in the ground but think they know everything because "college" or "CompTIA".
Some of these people get wind of what we're all doing though and then anything old becomes $$ regardless of what it is. I think that's where a lot of the ridiculous pricing comes from. While others would let the entirety of Computer Reset pre-LGR go for $1 if you drag it all away yourself - every last PC and part.
Then we're split as a group as well....just like any collective of enthusiasts....Some down to the individual. We're a mix of collectors, tinkerers, gamers, historians, old tech people with nostalgia, young curious tech people. Some prefer XTs, some prefer 486s, some PII, some P4. Some collect to play, some to make money later, and they're all valid and reasonable.
ITLDR think the real issue here is guidelines, standards, and mainstream awareness. To some people, it's just E-Waste, to others "hey, you can have my "problem"", and then some see it as a golden moneymaking opportunity with little regard to people who actually use these things
My DOS Boxen
85' Tandy 1000(a)- 8088/4.77, 640K, 8gb XT-IDE, TGA, 3-voice
89' GEM 286 - 286/12 w/287, 6MB, 1GB SCSI, 2XCD-ROM, ET-4000 1MB, SB Pro2
Creeping Net 486 - 486DX4100 WB, 128MB, 512K L2, 15GB+40GB, S3 809 2MB VLB SVGA, SBAWE64