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You can prevent your own economic demise by planning your markets properly. Here are some tips to help you prevent trading post disasters:
1. The markets you focus on need to have small demand with big profit margins or large demand with small profit margins. Here's two great examples. Vegetables sell very fast but with small margins of profit. If you post and are undercut, odds are you will still eventually get bought out. Armor sells slowly but with huge margins. If you get lots of undercuts, then you can cancel and relist for a small loss, but the large profit margin makes up for each undercut.
2. Always purchase with buy orders. There are very few exceptions to this. One of them being buying on Monday and selling on Friday, for a very select few markets. Or like special event items such as the greatsword skin.
3. Play a market by listing only 1-2 items at a time. Judge how fast they sell and then to determine how many more you will post. Some markets you never want to post more than 1 or 2 of the item at a time (like exotics) because canceling takes too much out of your profits.
4. Understand the Trading Post fees. Don't play blind!
5. Diversify. Even when you're testing, try to test a lot of stuff at once. Keep notes too, that always helps.
6. Never take my advice and follow it to the letter. Except right now with this rule. Kind of like the "every rule has a catch" rule.