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I saw a lot of questions on the reddit article recently about making gold that also mentioned this site. Since I'm mentioned, I might as well help out with some of the answers!
When you list an item for sale, you are last in line among any other items with the same price.
There is a 15% fee (5% listing, 10% after selling), which you can calculate by multiplying a sell price by .85.
Selling an item for 1 copper more than vendor price will give you a loss after the fees.
A 5% fee will hit you every time you cancel an item for sale. There is no penalty for buy orders. If you're calculating profits with the .85 rule, then it becomes the .80, .75, etc rule when you start canceling a lot. Be careful!
Here are some great tips for flipping if you don't know where to get started: