First, bookmark the list at tinyurl.com/RTGlist3
This list shows what is the current Rare Trade Good.
What is the big deal with RTGs? For a period of 6 to 9 days a trade good is worth a LOT more than it would be normally. How does it all work? Well you first have to know the current RTGlist. On the list, it shows where you can buy it from, collect/procure it or craft it even.
Now this is all great until you realize, you can probably sail to the next port over (who does not sell it) and sell it for a HUGE profit. Sometimes, the list will tell you it is a local specialty. Then it gets tricky. You have to travel two whole regions away before you can sell it and receive the inflated RTG pricing.
Now here is the key to RTGs, the are a global price. If it sells for 10000 in Seville, it will probably be the same price in San Antonio. It does not matter, most of the time. I know there is a slight difference when you sell it in your nation ports with taxes. Also because if it a global price, you can't go to another port and try to sell it (if you crash the market). You have to give it time for the market to recover.
I hope this all helps. Fair thee well friends,
Marjorie