In fact, by buying and selling amounts of goods and resources you do NOT need, you make the world go round.

You give those remote AI countries (especially in South America) the possibility to sell their raw resources and buy processed goods, while at the same time you gain from the merchant taxes. The AI does this as well, obviously, but we have noticed that if the player(s) don't do it as well the world economy stalls. You do not need to freak out about keeping stock of everything, after all, some abstracted lack of goods is pretty natural in wars. That won't be enough to destroy your population's contentment even for several months as there are three different categories of resources (food, common, luxuries) and within each type you can "steer" your nation's availability according to percentages (F4 screen).
No worries, if your ruler is a mercantilistic warlord he will be able to benefit by the "closed economy" and potential high tariffs in taxes. You do not need to simulate autarky by only buying and not selling.