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Anything orks love would work, though probably something highly combustible, explosive, and/or radioactive like fuel or mercenaries (yes, I believe orkish mercenaries fit all of the above categories). You could also try to buy loot and plunder off of them from ravaged worlds. Rogue Traders would also probably be willing to do a currency exchange if they were trying to scrape up enough currency for trading in orkish space. The problem is that ork teeth are perishable and are thus hard to find anywhere that there aren’t orks. As far as role-playing, you could have someone trying to buy up teeth in an area, someone cloning teeth to screw with the “economy” of an ork world, someone figuring out how to preserve ork teeth to do the same. They could be treasure, contraband, commodity, war trophy, cult focus, or plot point.