Why can't there be incompetent or just plain dumb rulers?
Good and effective administration might not be present in slothful or turmoil dominion.
When I'm playing a more or less "good" ruler I can find that it serves my purposes (and is thematically right) to just plain stall any deal. Sometimes even seizing the whole shipment without payment from the naive ruler I'm trading with. Perhaps that's what you get for trading with a diabolic demon lord whose rule is based on just corruption and lies.
I will of course face the consequences of that kind action, but it won't be so bad when I'm am mightier than they.
Other times I play a virtuous and humanitarian prince who only think good of everyone and cannot lie or deceive.