To sell commodities in trade you must enter an offer. You can enter offers on as many of your commodities as you wish. Offers do not guarantee that you will be able to sell the commodities. By offering you are letting the rest of the world know that you want to sell that commodity.
To enter an offer click on the "O" in the commodity row corresponding to the commodity you want to sell. A slider appears allowing you to set the quantity offered this turn. The quantity offered for sale cannot exceed the amount of ship cargo holds you have assigned to trade. You can see what this number is on the right side of the trade screen.
