The trade book is an informational tool. The only orders you can give on this screen are contained on the offer sheet itself. 

As each offer is presented, you can accept any number up to the amount offered, as long as you have cargo holds available to pick up the items. Change the number in the box and then click the accept seal shown on the offer sheet. To reject a deal, click on the reject seal. 

If you decide that you have purchased enough cotton, for example, check the box "done buying this commodity". You will be presented with no more cotton offers this turn.