Here are a few more principles to keep in mind, that are more specific to Pokemon than just any other money-making process or job:
1) If you're making a big deal, always set it up so that you get a profit margin reflective of your time put in. For example, I bought a massive lot of first ed and shadowless base for $150 a while back. So far I've spent about 10 hours total managing all affairs related to organizing and selling those cards, and my present revenue gained from it is around $500 after eBay seller fees and shipping. That's $350 profit, divided by ten hours, for around $35/hour. I have many cards left, and more hours to spend on it, but for the time being, that's a good margin.
2) Cooperate with other sellers when you can, because the overwhelming majority of the time, people here are friends in and out of selling. I've had more than one instance where I was in competition with a friend for a big lot, valuable cards, etc., but by keeping our cool and recognizing that we're still friends at the end of the day, we reached purchasing/selling situations where everyone won, including the seller.
3) Knowledge. The more you know about this market, the better equipped you'll be to make money. Had I been aware of the extreme value PSA adds to certain cards, I would be WAY richer than I am now.