Buy & Sell
Sell Goods you have in excess! Keeping huge piles of goods in your stash is not profitable. You might have a fortune waiting there which you could use to buy and build other things you need as soon as possible. Of course, you might have to sell these goods at a price below the standard price but just think about it. How much did these goods cost you in the first place? Probably just a few turns, uh? So go ahead, sell them. You're better off have 5,000 worth of something you can use now than 10,000 worth of something you can't use!
With your newly acquired trade credits, Buy Goods you need! You probably have several buildings running below capacity because of required goods shortages. You'll probably have to pay a higher price this way, but you gain several turns over the competition. A player who use trade a lot will be able to get a sizable fleet together much much faster than players who don't.
The Trademaster almost always buys and sells everything. He usually buys goods much below the standard price. Similarly, he also asks much higher than he should for his goods. By asking for and bidding at more reasonnable prices, players can outtrade(?) him. These prices are more attractive, hence we call them hot deals. As the game progresses, players will attempt to outdeal(?) each other, making hot deals even hotter.
Of course, at some point, you might not need to sell your extra goods right away. You might be able to wait a few turns to get a better price. So just advertise a sale! (Post a sell offer!). By asking for a price slightly lower than the current selling price, you take control of the price. This way, as soon as another player buys any of that good you advertised, its YOU who will make the sale and earn the credits!
So now we know players will attempt to sell their extra goods to the market. If nodoby placed a bid on that particular good, its the Trademaster who buy it. He'll do so at a ridiculously low cost... But perhaps you could make some use of that good, especially for such a low price. Well, you can do that easily! Just place a bid. Bid slightly higher than the current buying price and it's you who will get the deal the next time someone sells the good!
Earn credits by trading
There you are. You have all the tools you need to start earning credits just by trading goods. Just like WallStreet! :)
Here's an example of how you can earn credits. Notice how much higher the price of a Laser Cannon is over the price of the components required to build it. If you buy the required components (a Comp. Struct. and Diamonds) from the Tradepost, build the Cannon, and sell it back, you earn a few hundreds credits... Cool, uh?