@Thomas,
Thomas wrote:You don't compete with these morons, because there's no communication between buyers about the prices they got.
Correction: There's no communication between
sellers about the prices they got.
Let me take this opportunity to describe the typical setting in which the haggling happens. Imagine some small booth (four by twelve foot perhaps) that is divided in the middle by a bullet-proof, bank-counter-like wall of glass. Buyer and seller face each other through that wall of glass. In the wall to the buyer's back is a door to "the back office", where the buyer may go to "consult with my boss". Behind the seller's back is a door to the business's general waiting area. That door is closed during the negotions. The haggling, as well as the final deal, are fully private.
Well, not
fully private. Security cameras are everywhere in the building, and as I said, one of them is on the seller's side of the booth. So when the buyer goes to "the back office" to consult "his boss", he can look at the monitor connected to the camera, and listen in on the buyers' conversation. I can't prove that buyers actually do this---but it's an obvious opportunity, and they'd be foolish not to take it.
Nevertheless, there is complete secrecy between sellers and other sellers. The diamond district is not the New York Mercantile exchange; there is no unified market and no consistent market price. And because there's no transparancy between sellers, buyers can pay very different prices to different sellers.