I've often downplayed the "buy low, sell high" aspect of the money-making game, instead focusing on using trade skills to create value and profits. There is, however, a lot to be said for brokerage. It can be much riskier but has a very good profit per hour when you win your bets. Unlike a game of cards, though, the odds are not stacked against you.
There's one key fact that can stack the odds of brokerage in your favor: People charge less for items in a trade window than others would pay on the auction house. The best strategy to take advantage of this is to buy low and sell long. Why long instead of high? They actually amount to the same thing, but the key here is that instead of immediately trying to find some buyer who will put more money into a trade window than you, you list the product on the auction house.
The auction house is the great differentiator between people looking to make a quick sale and those willing to wait for a high margin. If you see someone selling something in trade chat, chances are that no matter how much they're asking for, they're expecting a low-ball offer and are willing to sell somewhere in the middle. So what is it about the AH that makes stuff sell for more?