Try to find bulletin board missions for trading, will give you more money, and sometimes you don't even have to buy the cargo and/or does not take up cargo space.
And I don't mean this to be sarcastic in anyway, but I found trading to be too much work for me, and I started doing bounties at nav beacons and resource extraction sites and I have made several orders higher more money than I ever would have trading. And it is not nearly as boring, but more risky. However, that's just my play style, yours could be different.