Every day the population consumes a certain amount of goods of each type. You can sell only the excess of goods. In other words, the amount of goods manufactured must exceed the number of goods consumed to make the excess goods be stored for further selling.
Costs of products vary depending on the country buying them. If a country does not have enough supplies of this kind of goods, it is ready to pay more than other countries. The richer the country, the more it is ready to pay.
To increase the number of excess goods, you can construct plants.
Don’t forget that after you conquer a country the number of excess goods you have stored may decrease. You have annexed a country the economy of which was ruined by the war, and your industries will have to cover the needs of the population of your new territory.