Just a few tips, since the big issues I wanted to mention already were:
1.) Germany is going to trade a thousand times better if you go monarchist, than if you are fascist; as nonaligned, you're going to get worldwide trades from everyone from China to Argentina to Greece.
2.) Timing and ideology are critical (democratic/nonaligned are the best traders). Too early, and people still have their own steel, too few mils to need more, and too few civs to buy much. A few years in, you see more getting bought. This is why Germany does poor in 1936 with trade, but much better in 1939 (when there are more fascists and Axis nations, and more steel demands).
3.) Steel is the main resource the AI always buys, because it uses it on everything and few minors have much (here and there random nations like Greece and Chile have some, but mostly its concentrated in the major powers). By the late game, there's a clear deficit of steel throughout the whole planet.
4.) Aluminum and rubber are sometimes worthwhile, while oil is a tossup (since oil gets bought for a little while, then stockpiles fill up and it stops). A lot more minor powers have aluminum, however, and rubber isn't a good sale while France, Malaysia, and Indonesia are still at peace. Tungsten gets better over time (early its just a bit for artillery and AT), but demand never goes up that much, while chromium is basically not used by the AI (they don't build jets, heavy tanks, or capital ships...which basically represents every possible use of chromium).