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EDIT: The person who emailed me was "Badaza."
Once you start to master the basics behind profiting on the trading post, particularly the math of undercutting in my Damn Gold Guide, then it's time to optimize your game play. A super simple and easy to duplicate method is the "250 Item Buffer Rule." I didn't come up with this method as it was emailed to me, but for the life of me I cannot find the original email. If you'd like to claim credit in the comments below feel free. Sorry :(
OK, back to the idea though, and it is a brilliant idea. I like it because it's simple yet teaches a valuable lesson: you can sell with more intelligence than just 1 copper less than the cheapest seller.
What you do is take the first 250 items you see listed on the trading post and ignore their prices. This only works for crafting materials and other relatively fast sellers (unlike armor and weapons). Here's an example since it's much easier to show than to explain:
Look at these onyx slivers.
All I would do now is undercut to 3 silver 77 copper. While it will take a bit for my item to sell, I will make far more than if I were undercutting the cheapest listing. Multiply this by a 100 onyx slivers and I'm earning around 40 silver more than if I sold at 3 silver 35 copper (after fees).
To go from the selling page to the item's individual page on the trading post, you need to open up your inventory ('I' key) and right click to "Buy More at Trading Post". It's clumsy, I know, but the only way.
See if this method will work for you, but be careful! Start small and leave a lot of items in your bank. If you aren't aware of the general prices of items then you might be listing your items at very expensive amounts that the price rarely reaches. A good reference would be Guild Wars 2 Spidy before posting. Odds are you will make enough back with this method to easily cover any mistakes learning it. Thank you to the purpose who suggested this wonderful tip, and please email me again so I can give you credit or respond in the comments!
It's a good habit to get into, checking more than just the lowest seller's price for your item, and it will sometimes reveal amazing deals. Like when someone lists an item for one copper more than the highest buy out price!