It may not be a good idea to sell items until your industries are a bit more developed, but you should immediately begin to buy commodities, especially resources. 

To bid (attempt to buy) a commodity click on the left-side brass slider in the same row as the desired commodity. You will find these sliders in the Orders column of the screen. When the slider appears it will have the word "bid" written on it.

You are limited to four buy bids in any turn. Additionally, if you decide to offer something for sale, you will not be able to bid on the same item in the same turn. 

You can change your bids at any time until you end your turn. If you leave this screen and return later your bids will be saved.