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There's one simple, golden secret that I use on the Trading Post.
If you buy one of something and flip it successfully, then you should buy two the next time. After two are flipped then you should buy 25. After that? 50.
This golden rule of 1,2,25,50 is how I make a ridiculous amount of profit flipping. Many complain that they are spending all day finding things to place buy orders on. Well, after a while, you should just be able to look through your "Items I've Sold" list and pick out something you'd like to sell again.
Eventually you should have a little excel spreadsheet (that's what I use anyway) and record how many of the item you last tried to flip. When you look at your "Items I'm Selling" list and don't see the item, then you mark in your spreadsheet so that next time you attempt to flip one tier higher. The tiers I use are 1,2,25, and 50, but you can use whatever you like.
Using the my transactions tab correctly is essential to this strategy. You should already be used to looking up the four lists that Guild Wars 2 tracks for you about items on the Trading post. Namely: Items I'm Selling, Items I'm Buying, Item's I've Sold, and Items I've Bought.
Have you neglected to use any of these four so far? Perhaps now I've convinced you to take a second look at the my transactions tab?