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There are two titans when it comes to flipping for gold: Vegetables and Armor. This dynamic duo are so very different in terms of the style of how you play them, but both have phenomenal ROI (return on investment). I can invest 3 gold into low level armor and get back 8-10 gold reliably, but it can take upwards of 3 days for them all to sell. I also have to relist on occasion if there are too many undercutting players, so that takes a percentage out of the possible profit, but they all still sell eventually. Vegetables, on the other hand, sell very quickly but you only get back a small fraction of what you invested. Often times I am putting up to a silver in per vegetable in order to get back 1 copper a few minutes later on each one.
Both markets play out very differently, but you still end up getting similar returns on your investment in terms of time and cost. Have you tried flipping either of these markets? What was your experience with them?
On the forums I gave a description of how I sell armor in bulk, including cancelling quite frequently due to the large profit margins. If you stick to level 1 - 35 armor of all types (not just the best quality available per level) you will find insanely profitable flipping margins. This enables you to do the cancelling I have described, even if you're cancelling a full stack of 250 items! Vegetables basically follow the same principle as armor, but you do not cancel them at all. You must wait for them to sell, but usually they move so quickly (assuming that you go for the minimum amount of profit) that it doesn't matter. Grapes, for example, are a great way to get 1-5 copper back in a few minutes per grape, especially during peak play times on fridays and the bulk of the weekend.
So, have you tried armor or vegetables? I view them as the best ROI in Guild Wars 2 at the moment, and I'm sure you will as well if you give them a try.