Except in real life you are limited by demand, which usually translates into either small, high margin trades or bulk, low margin trades. Yes, bulk high margin trades happen, and it should remain a possibility in the game, but the way things are, it's incredibly easy and somewhat frequent to come across trades that will set you up for credits for practically the rest of the game.
Well yeah, true, but I was too arsed to go into any of the details.