I started wondering about this while surfing the FS forum looking for nothing in particular. My last purchase there was for $250, and it wasn't one of the brands that appear frequently. It doesn't cut any better than my $40 Calypso Jr but it is beefier, better looking, and waaay cooler. I've been thinking about getting one for years and finally did. And, as KWM suggested, the seller was a student who needed some dough to get to the SHOT show.
On the other hand, I've bought one or two of the knives, new from a dealer, that frequently appear on the FS forum. They were definitely overpriced when new, at least I thought so upon receiving them and still do. I would never have bought them if not for threads singing their praises. Not that there was anything wrong with them, but they just weren't what their admirers said. But I used them. One served as a trunk/camping/chopping knife for a few years. Lost it, and replaced it with a similar knife I found in FS, but one of the brands that doesn't come up so often and was about 1/3 the price of the knife it replaced.
Anyway, I buy expensive knives and use them too. Some of them were worth what I paid and some were not. On the other hand, there seem to be a significant number of people who buy expensive knives and then try to sell them NIB. Some might be collectors or frequent traders, I understand that. But I would guess that a lot of them, maybe even the vast majority, are guys who shelled out way more retail dollars for a knife than it's worth and realized their mistake pretty soon after getting it. None of them are going to post a sale thread and say "I paid way too much for this thing, you wanna take it off my hands?", but when the same brands or models pop up over and over, well, I get the message.
Randy, one of my coworker got himself a Softail for Christmas, brand spankin' new from the dealer. I just smile when he says what a good deal he got. Two months later he's already saying that he should be able to sell it for what he paid. Hmmm ... this sounds a lot like the situation I see on the FS forums from time to time.