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Whenever I'm solving a bug in computer code, I tend to hit walls where I say something like "This has to work, why isn't this working?"
The same verbiage is said by people who are failing to make gold on the trading post. "This has to sell, why isn't it selling?"
After saying these words, I always take a step back and say: "Well it didn't work, so my premise is wrong. It obviously isn't guaranteed to work."
Once you make that acknowledgement, then you can work towards figuring out why assumptions that you based your statement on were incorrect. You run through the most obvious things first, like is the external hard drive not plugged in? Definitely assumed it was, but now I have to go back and check all my assumptions until I find the real problem. Or at least close off some wrong "doorways" until only the correct one is left.
This kind of logical reasoning can be applied to the trading post as well. One thing you should never assume is that you know how to play a specific market before having any experience in it whatsoever.