Buy off the world market at the minimum quality. This is much better. Any higher than the minimum quality is a waste since these won't increase the value of the goods your selling (as determined by the average supply quality).
Don't bother trying to buy products that are available on the world market for everyone at any time where the precise quality you want can be selected.
Save the trouble for "limited" products like Plutonium, Cruise Missiles, and Strategic Bombs.