A basic strategy of trade is wise use of your merchant marine. As each offer is accepted, the amount of merchant marine remaining this turn goes down, as displayed by the number beneath the ship icon.
Commodities are always traded in the order shown on the trade screen and in the tabs on the side of the trade book.
This means that if you desperately need iron, for example, you may need to stop accepting all the wool you can get, to reserve some of your limited merchant marine for the iron coming later.
You should use the trade book (iron tab) to look at the market in iron. If one of the nations selling iron shows your flag as its top bidder, you will definitely receive an iron offer this turn. You should save some merchant marine for it.
