Dark Iron Bars are not bop. It is like any other type of metal in the game. The reason you probably don't see Dark Iron Bars in the auction house too often is it is a hassle to smelt dark iron. Dark Iron Bars can only be smelted at the forge deep in BRD and to make 1 bar of dark iron you need 8 ores of dark iron. Also dark iron ore by itself is used for reputation quests, so people will most likely sell the ore itself since it could be used for more things than the bars.
Hope that helps.