We Need More Writers!
I would say that the number cause of issues on the trading post and investing in the real world is focusing too much on a single moment in time. There are three possible ways this mistake can occur.
1. You think about a past price for an item and hope for that value to return.
2. You look at the current price and assume this is the value it has and will always be.
3. You speculate on a price that hasn't happened yet and assume it must occur.
By far, the second one happens the most. Let's look at an example:
Oregano leaves are 2.07 silver listing and 1.47 silver buy order. That's a profit of 29 copper per flip. You're probably pressing o right now to look at the trading post and search for Oregano Leaf. If so, then you fell for the second way you can perform the biggest mistake on the trading post. What you don't realize is that Oregano leaves have been dropping in price pretty steadily since march when they hit a peak of 3 silver.
Click this link to open a new window for Oregano Leaf and play along with what I'm saying.
By looking at gw2spidy and turning off all the graph lines except for Sell and Buy Listings Raw Data, as well as setting the Zoom to All, you can see this information. Last year the price remained steady throughout the summer months and I expect that the same will be true this year. However, we probably haven't hit the bottom for this market yet. So if you buy a bunch now, the price may not be flippable for a profit by the time you go to sell them.
Please do yourself a favor and study the guild wars 2 spidy charts for markets. Besides the Sell and Buy Listings Raw Data, you should also understand the Sell and Buy Listings Volume. These two lines will show you a trend of how many items are being posted on the trading post. If the blue lines trend upwards while the red line remains approximately the same (as is happening with the Oregano Leaf right now) then the price will likely drop. If you turn on all four of the lines I've mentioned, you'll quickly notice a pattern of when prices drop and rise based on the Sell and Buy Listings Volume.
I don't find the other graph lines as helpful as these four that I've mentioned today, so I recommend sticking to those while you learn.