I've been reading this thread with a lot of interest too...
I have only very limited experience with selling vintage, but at least one of my ebay sales of a dress that didn't really fit me went to a dealer, and I think I got a fair price for it - but I'm sure the buyer wouldn't have bought it at that price if he didn't think he could sell it at a higher price. But that's life... I can live with it. If I had sold it at the flea-market I would have gotten less for it.
I've experienced a lot of things in my field of fashion doll (Barbie) collecting and have developed a philosophical view of these things.
What I've seen happening often at doll fairs was that one dealer would buy a dressed doll from another one, but actually only wanted the doll, or only her outfit. And for sure, the buyer would undress the doll and then try to sell the dress/doll he hadn't wanted right away at his stand... and maybe not even so cheaply. It has happened to me we things I sold at our club's fair - but then I have done the same thing too, although never directly at the same fair :bouncy: .
Karin