Oceans and Empires Trade Market Details
First of all, think about the Trade System as nothing more than the Stock Market. Prices rise and fall, fluctuating almost constantly. Much like our Stock Market, it can be very confusing to some players in regards to the system. Though it is easy to learn and you can start making some nice income soon enough. This is accomplish through your Trade Warehouse located to the left of your Docks and to the right of your Shipyard.
One of the most important aspects of being successful with the trade system is your positioning within the map. It is best to locate yourself between several (4 or 5) cities with a range of items having a nice profit margin. Once you have centered yourself, your Trade Warehouse becomes priority. Go to your Trade Warehouse and check Price Info for Best Routes.
For Example: As you can see in the picture above, Lima has Salt for 48 Silver. Of which you can sell in Mogadishu for 94 Silver. This generates a 96% profit. Anything below 30% is not worth the travel time.
Trade Warehouse Storage
Lastly, you will need to have a hefty storage size in your Trade Warehouse. Only so much resources can fit within your storage. You will not be able to purchase 100k worth of Salt from Lima and ship it to Mogadishu if your storage can not hold that much. To increase your storage limit, upgrade your Trade Warehouse.
Source: Oceans and Empires