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Reference: Damn This is A Gold Guide
Start Here: Flipping Guide Part 1
You should have a few items to sell after one day of waiting for buy orders to complete. Some will be more successful than others, so don't worry about poor performance in any one of the markets you chose to try. Just make sure that you're documenting the following information:
Buy order price when purchased
Sell order price when sold (you're doing this part today)
Over time you should catalogue multiple examples of buy/sell prices. Always pair the most recent purchases with their corresponding sell orders. So you might do something like the following.
Cotton Scrap 30 c / 39 c
Cotton Scrap 32 c / 42 c
Cotton Scrap 29 c / 38 c
This will give you a better idea of how low the item has reached recently. Next time I go to place buy orders on cotton scraps I will ask for 29 copper even if it's currently 32 copper, knowing that it will likely fall that low again at some point.
Selling Your 3 Items
Your first item to sell is the piece of low level armor. Right click on it in your bags (after taking all from the trading post), then select Buy More At Trading Post. This will bring up the piece of armor in the trading post window. Click on the armor. You will be able to see all the current prices displayed on the right hand side.
You can see from this example that there is a "wall" of listings at the 1 silver 59 copper mark. If I'm willing to wait out the other listings, I can sell at 1 silver 58 copper in a realistic amount of time. However, if my profit is already quite large, I can sell at 1 silver 49 copper quickly. In the case of low level armor, I usually take the faster profit option.
Next up is your cooking ingredient. Check what you bought the item for (shame on you, it should already be documented in excel or even just a piece of paper on your desk) and divide by .85 to determine the price you need to make a profit. Also right click on the item to buy more at the trading post so you can check for any "walls" of listings. Unlike in the case of the armor, I usually post just under the nearest wall of several thousand items. Cooking ingredients sell by the 250's, so a few thousand moving per hour is no big deal.
Having a large listing of prices you've bought and sold the item for is quite helpful at this point. You can also read guildwars2spidy charts as I show you in the linked post. Also check out GoldWars2 for even more statistical information and guidance.
Last but not least is your crafting material. Similar to cooking ingredients, you will want to look for the nearest wall of listings. You also need to look into the possibility of mystic forging or even salvaging the item for a profit. That takes a little more effort and research, but is quite worth the effort!
So here's your three items in a nutshell:
Armor - Sell quick for 1 copper less than lowest price, assuming that the lowest price is not some outrageous value. If there are only a handful listed too cheap, list above them.
Cooking Ingredient - Go for 1 copper less than the lowest wall price.
Crafting Material - Go for 1 copper less than the lowest wall price OR salvage OR mystic forge.
Ready for the next step? See flipping guide part 3.