On the trade screen you tell your merchants which commodities to try to buy and sell when the world market convenes after the end of the turn. 

There are no guarantees on the world market. Your empire might need coal, for example, and have plenty of money, but if no other country is offering coal you will not be able to buy any. Likewise if no other country is bidding on the commodities you offer, you will not be able to sell them.

Use you powerful industries to take advantage of the minor nations: buy their cheap raw resources and sell them back expensive finished goods. 

This turn you should consider changing the bids already entered if there is something in particular you wish to buy. You should decide what to attempt to buy based on what resources you lack.