When I play Settlers of Catan, I often find myself rapidly bored by the trading component of the game. This is normally because of one or two players who insist on endlessly protracted negotiations for any item. These always start at ludicrous extremes ("I want a lumber.", "I'll give you a lumber - for 3 sheep, a brick and two ore!", "No way!", etc. etc.) and take forever to converge (if they ever do). I find this type of thing can quickly suck the life out of the game. This is a shame, as, apart from that aspect, the game is quite entertaining.
How do you handle this sort of behaviour?