I decided to focus my efforts on growing my trade business.
In the trading post you have two features:
1) I am looking for:
2) I offer:
Then you can choose building materials or any of the luxury items and a search radius (assuming your trade port is big enough to trade outside a one island radius).
I am looking for:
These are items that everyone has up for sale and is fairly self explanatory. If you want something simply click the drop down arrow, select your item and click Find Bargain.
If you find your item for a price you are willing to accept, simply click on the Trade Icon.
In the next window you can select the maximum price you are willing to pay for an item. I personally have not had any luck attempting to offer less than the asking price for any items. Then put in your quantity and click on Buy Goods. Now the ships have to travel to the destination, the goods are not bought until they reach the island, so the downside to having a large trading post is that your ships have a long travel time and may miss out on any really cheap goods you find.
Now the I offer is a section a few of my fellow 8th members had not looked at too closely when I started telling of some of my successes
These are items other towns are willing to purchase luxury goods at and the price they are willing to pay for said items.
The search feature works the same as the "I am looking for" feature. Most of the searches you see in here are people looking for items at an absurdly low price. 1,000 wood for 1g, 400 marble for 5g, etc. but once the towns around you start to develop the offers become more reasonable and if you have some excess inventory to sell this could be the place to go.
Also you can contact the person looking to buy the goods through diplomacy and see if you can also work out a trade if you are needing a good and the prices of what you need are too high in your area.
Advanced trading:
Once you get your trading post up a few levels, you can start looking at trading for profit.
I always go through and look to see what prices good are selling for in my area. Typically they are 20-30g per item, but every now and then I will see an items pop up for 5-10g, I immediately switch to the "I offer" section to see what people are willing to pay for that same item. As I said before usually it's a very low 1-5g price, but if I see someone selling it for 5g and someone willing to pay 10g for the same item, I'll send a ship out to see if I can get the items for 5g. About half of the time the island is too far away and my ships get there too late to buy the item, but if my ship picks it up I go back into the "I offer" section and ship out (assuming I have it in inventory and don't have to wait for my trade ship to get back) the goods to their island to try and make the deal.
This works because a lot of the smaller trading posts can not see the deals that may be just out of their reach. I starting doing this around a level 3 or 5 trading post, currently I'm upgrading to level 7.
Best deal to date: I bought 100 wood at 1g each and sold it for 10g each. The person selling the wood apparently was trying to put in a price of 100 because he immediately listed another 400 wood at 1g and I sent two ships to pick it up, but by the time they got there he had changed the price to 99g per pc.
NOTE: It is easy to tie up a lot of tradeships doing this so make sure you keep track of how many ships you have out and leave some in town if you need to go pillaging or transport goods between your colonies.