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When items rise on the trading post, there are often linked markets that take a little longer to catch up with the new prices. One great example of this phenomenon is the rising price of cloth. Linen scraps, in particular, have risen significantly over the past few weeks. What I did to profit from this was not buy Linen Scraps, but instead, the containers that when opened have a good chance of producing the cloth.
On the forums a week ago I outlined exactly which bags I purchase for this purpose, as well as additional strategies branching off from this method. I'm not going to release this information to the general public, but I'd be happy to give you the most important piece of the puzzle... this page on GW2 Wikipedia that outlines what each container can produce when opened.
A little research goes a long way for figuring out which bags have the best chance of earning you a profit. You also have to factor in the amount you lose when you get less than optimal results. Even in the worst case, there is a bag right now that results in top tier items nearly every time. So you'll lose a silver or two when that happens, but then you'll make a lot more when the right items drop.
Create a little spreadsheet for yourself where you list each bag, the results that can happen, and assign an average value to each one. So if a bag has a chance to drop 1-3 linen scraps, count it as 2 scraps and assign a value such as 12 silver to that result. Let's say there are six other items in the bag that can drop, you just have to record those values as well and then figure out your percentage chance of making money from 1 try, 10 tries, 50 tries, etc. Get the math down after collecting the data and it's a real breeze!
More great examples of prices not catching up include but are not limited to:
High level rares that salvage into ectoplasm.
Low level basics that salvage into copper.
Mini pets that you obtain from black lion chests (more complicated).
AND MANY MORE!!!