How to trade
To begin trading you need a Comsensor manned by at least one Targ and a Star Dock. Having a Cargo Hold is also a good idea. The only exception is Arona, who seems to be able to trade without these things.
Once you have the built the required Buildings you will begin to get messages from passing traders. Clicking on the message will tell you who the trader is, what goods they have to sell, and what they are looking to buy.
If you accept their request they will begin to dock at your Star Dock. When they are docked and ready to trade the message icon will turn purple.
Clicking on the message icon or the ship in star dock will open the trade menu. Here you can buy and sell cargo by clicking on its icon. A description of each element of the trade interface can be found here.
Buy low, sell high
Each trader will pay more for certain goods and sell certain goods at a lower price. This information is usually included in the contact message. Arona sell most things at higher prices but also has access to some rare items. Further information regarding what races will trade is available on each Peep's page.