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In Diablo 3 they're called commodities, but in Guild Wars 2 they go by many different categories. In essence, a commodity is something that you buy and then "use up" or "destroy." Crafting materials, cooking materials, dyes, etc. These are all examples of commodities.
At the moment, I'm following the advice that can be found in my flipping strategy guide to turn a large number of commodity markets into my own personal playground. Simply put, the current price that items are selling at does not concern me. All that matters is how low I've seen the buy order get in the past. I can confidently place buy orders on items knowing that the buy and sell price will go down/up at some point to values I have tracked previously. If you aren't aware of websites which can help you do this, I would recommend Gold Wars 2 or GW2Spidy. But in the worst case scenario, just keep track of the values like I showed you how to do in the flipping strategy guide!
A lot of items cycle into the top 10 traded list on the trading post. When they make it to the top ten, they become more visible to the millions of trading post players, and so their buy/sell orders start to get closer and closer together. Once people stop trading (because it's no longer profitable) then the item falls off the top 10 traded list and returns to a profitable margin. This cycle (whether I'm right on the cause or whether it's a combination of many factors) is predictable!
All you have to do is write down the items that make the top 10 traded list, and then see how their price changes when they fall off or get onto it. You'll be amazed!