At the beginning of the game you should immediately begin to buy commodities, especially resources your Empire lacks.  You should sell commodities when you need money and when you can afford to expend your valuable materials in this way.  All the materials your industry can produce can be used either to assist your development or sold to make money.

An attempt to buy a commodity is called a bid. To bid, click on a “B” icon in the bids column.  A gold wax seal confirms your  action.  You are limited to three bids in any turn until the technology of trade fairs is discovered by your Empire. Additionally, if you decide to offer something for sale, you will not be able to bid on the same item in the same turn. 

An attempt to sell a commodity is called an offer.  To offer, click on an “O” icon in the offers column. A red seal confirms your action. Enter the maximum quantity you are willing to sell using the slider that appears. You can offer for sale anything that you have, unless you have insufficient cargo holds to make the delivery. You cannot offer and bid on the same commodity on the same turn.

Once you set your offers and bids, you need to return to this screen only if you want to change them.