you're forgetting the effect that demand has on the selling price of a good. just in terms of number of prospective buyers, demand: Ebay > SF B & S > SF thrifting thread. i'm sure you understand what effect this has on the selling price of a good.
given that, i don't understand why flippers who are looking to maximize profits even bother posting stuff here. why not just go straight to B&S or ebay?
EDIT: i meant this in a strictly "economics 101" sense, not in a "you don't belong in my treehouse" sense.