Not to mention iron & steel crafts, armor, weapons, anvils, earrings, flasks, toys, bolts, pillows, instruments, pots, pans, kitchen knives, and any other ferrous metal items the caravan can bring. If it'll stick to a magnet, buy it. Buy it all and melt it down.
Set up stockpiles near your magma smelters that will take the iron armor and one to take iron & steel finished goods (and whatever other crap you want to melt). Have them take from any similar stockpiles you have that aren't limited to just the iron and steel. So from the depot, the stuff gets taken to your general finished good pile that is closest to the depot. Then Urist notices that this iron crown (masterfully decorated with silmarils) can also be taken to the stockpile that takes from the general stockpile, so he then takes it down near the smelters, where it is melted down.
You can either mark stuff for melting immediately after you are done trading by using 't' to look at the objects in the depot and scrolling down and hitting 'm' as you get to each one you want to melt. This gets tedious though, since there is usually a lot of stuff in the depot at that point. You can also wait until the stuff gets to the "stuff to be melted" stockpiles and use d-b-m to mass mark it for melting.
Then just have each smelter have a permanent "melt object/R" rep[eating order. As your manager assigns other smelting jobs, the melt order will stay on each smelter and cycle its way around the list, so your smelter will spend 10% of the time melting stuff. If that's not enough to keep on top of the incoming iron ore wonderful iron stuff from the Mountainhome, just add another repeating melt order to each smelter.
Keith