Here you can buy and sell goods to other Castleville players.
To sell... select the good that you want to sell and how many of this good you wish to sell. A suggested asking price will come up (this is the price that Myra would buy it for) and you can adjust this to be cheaper or more expensive. Now we "set sail". The Sea Trade port will now have a flag on it indicating that you have goods in the trade. You can visit this building to find out how long you have to wait for your ship to return.
To buy... if you are at a level that allows you to buy these specific items that other players are selling, then all you have to do is select the item you wish to buy and viola. Keep your eye on the cost of these items before you purchase them because once you select the item there are no returns.
The third tab on the buying screen are all items that your alliance (level 10) sell.
The Selling on High Seas quest will help you practice navigating the Sea Trade.