Concretely, from years of strip and flip, I'm at the point I wouldn't even bother buying a 75€ XT M739 equipped bike.
By the time and cost of getting it in the man cave, inspecting it all, cleaning it all, presenting it all, dealing with dickheads, the pleasure and financial incentive is now lost.
Dealing with French bikes (where I am home and are abundance) I would never pick anything up more than what the weird ass 3 or 4 speed freewheel is worth. It is that sad to keep level headed today for dealing with that kind of stuff.
The only thing I think that actually maintains some flow of goods is disparity between rural areas and urban areas and the notion of "click to buy" and be gratified instantantly but even that is laced with complications when considering values.
A few days ago (in France) I dumped a massive 1952 made ceramic sink at the tip. Made in England. In excellent condition with no chips. Here in a deep rural I can't even offload for a cow to eat from it. The guys at the tip to help me offload it said the same, in a swanky city it would be in a bathroom costing a lot for a unique vintage item to show off. Proper country house renovation a la TV vibe to it. A UK house renovation program would make 15mins of broadcast time on such a thing. A trendy influencer on the boobtube raves about such things, but they haven't been arsed to get a driving license and certainly can't be arsed to collect such things. The new generation I think wants it delivered on the door stop and be able to return it if they are not "happy" with it.
When we are talking about bikes, especially older bikes, as private sellers I think we are up against it to appeal to a new crowd. The hassle and price of posting big bulky items I think weighs on the decision too that effects our market.