I'm almost done with fixing the trading system... It was definitely broken.
I should add per-planet trade controls (i.e. 'only sell food on this planet'), but this change will make consumer goods more important on low society planets, which should keep the resources that you want concentrated were you want them (which was always my intention).
I will be adding a per planet 'happiness index' fairly soon as well, which will take into account things like new construction, foreign trade, etc etc.