Besides this video, I thought it would be best to include a similar, written blog post on the subject (for those people who can't watch the video at the moment).
I would argue that the most profitable markets in Guild Wars 2 are also the most violent. By violent I mean they look like the following price graph:
Notice how the price goes up and down over time? That's good. The faster this pattern occurs and the more often it repeats, the better. To the uninformed consumer, this kind of market is annoying. They never know what the price of the item they want will be. But for people who are placing buy orders and listing the won auctions when the price goes back up, these fluctuations are a beautiful thing.
Instead of buying whenever the price hits rock bottom, you should aim for somewhere a little closer towards the middle. Same goes for selling, but in reverse. You want to maintain a profit but not wait until the price gets absolutely to the tippy top of the wave on the chart.
Guild Wars 2 is unique in that you can place buy orders, thus acquiring cheap goods without even being online. But the way to ensure that your trading post is full of ready to sell items each time you log in is centered around the fact that you will have a difficult time buying items for the absolute lowest amount that they can trade for. You're much better aiming at 50%-70% of the price point you want to sell at. The same is true for selling, you won't sell many for the max, but rather, you'll want to aim for somewhere 10%-20% lower in most cases.
Day trading and weekly trading are a lot of fun in Guild Wars 2, provided that you're not in a rush and that you have some experience flipping items for a profit. This strategy does not work the same as the one I outlined in Damn, This is a Gold Guide. The biggest difference being that you want to place buy orders well below the current highest, instead of just overcutting by a copper.
You therefore need to pay attention to where the biggest walls are, as they will often be the top or bottom of the price curve. This happens because of all the people following the other strategy. The price gets to the bottom or top, lots of people place buy and sell orders, but they don't get purchased. Therefore a wall forms of items that just sit there. It's a really telling mark for how high or low the price can get.
Tomorrow I will announce 50 markets that are extremely profitable if you play them the way I've been describing in this post. If you want a head start, you can join the forum.