Gromril - dwarfs would never, ever sell gromril, a non-dwarf having anything gromril on him is a kill-on-sight deal for the dwarfs

Warpstone - resource that you'd only sell to other skaven, game doesn't accomodate this

Slaves - resource that you'd only sell to races that practice slavery, game doesn't accomodate this

Ithilmar - too rare and precious to the Elves to be a general trade resource

Elven steeds - HE horses are really special in the lore, elves would never pack them up on a boat to sell them, some get sent to Bretonnia to replenish the genetic stock of Bretonnian horses, but that's a special case

Gold - I feel like gold gets minted into coin, not so much traded, and that's why CA didn't make it a trade resource in the first place

Other people will make mods that add these, sooner or later, so you can use them then.