I've done a bit of buying and selling since this thread started (N-3 +1) my experience has largely matched my opinion above:
Sold on Ebay, Surly Ogre frameset, two years old, used one year for Deliveroo so a good few marks and scratches but no real damage, honestly described with lots of detailed photos, bid was about 60% of what it cost, though obviously Ebay had a slice, happy with that, probably influenced by the price having increased and they're currently hard to find in stock.
Sold Facebook, my Hewitt Cheviot frameset, almost immaculate from a respray a few years ago. Didn't think it was going to sell, then three people wanted it in the same week! £150 didn't seem a lot, but it's an 18yo frame and I've had more than my monies worth.
Mercury damaged repainted frame and new Thorn fork. Advertised lots of places and sold via a forum, because of the bikes history I wanted to sell face to face with the buyer rather than an internet stranger. There was no interest over a couple of months as a complete bike with Alfine, a couple of enquiries for the frameset, just about made back the money I'd spent on the fork and respray, still glad I did that rather than scrapping it.
The Hewitt had lots of bling parts, Paul Components brakes, Nitto bars, Campag bits... those all sold well, I got more from stripping it than if I'd sold complete, goes back to my earlier point that enthusiasts like to pick their own components. Only disappointing sale was some quality Audax wheels, only a couple of hundred miles and they fetched less than the cost of the rims and it took three goes at Ebay to get that.
With the exception of the bling and wheels I'd adorned the Hewitt with, I'd already had my value out of these bikes and bits, what I got for the sale was a bonus.
What sometimes makes me sad is when I see a bike advertised at a fraction of the purchase price, when it's only done a few hundred miles. Good news for the buyer of course, but in some way, for some reason, the original owner probably hasn't fulfilled what they'd hoped to.