Some brands or makes of bike seem to sell for very high prices, but if you know what to look for and don’t follow the herd, some amazing bargains can still be found.
Certain makes and models seem to sell on their reputation. A Dawes Galaxy seems to be the default go-to choice for a Tourer, with the Raleigh Royal also having a following. Both very nice machines, but are they necessarily that much better than everything else as their values suggest? Generally both tend to go for higher prices used than other, less well known, but similar quality bikes.
A lot of cyclists are ultra-conservative in their tastes, and will only buy what they are familiar with. To buy a name you haven't heard of, or a plain frame with no makers decals whatsoever, can be a leap in the dark. The advantage the well-known bikes have is information availability; I can easily find out details like frame geometry for a popular off-the-peg bike, therefore I know roughly what to expect in terms of ride feel.
For a lesser known or custom frame I am left with looking at a picture thinking "is that a 73/73 frame or is that head tube 74? Would it be too twitchy? Is the ride going to be harsh on crappy road surfaces? What size tyres am I going to get away with fitting?"
When buying online you are taking a bigger chance than a face-to-face sale where you can "try before you buy".
I agree there are plenty of bargains out there, but I tend to go for stuff that is familiar to me but badly described or poorly pictured, rather than venture into the unknown. My own experience says even bikes that tend to sell well can be bought as bargains if you are patient, with an element of luck. i often see two near-identical bikes sell at wildly differing prices within days of each other. It just depends who else is interested. A Trader looking for stock to resell won't pay so much as a private punter, because he needs his profit margin, so will bail out sooner. A couple of determined private buyers getting into a bidding war can result in a silly price.