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Step 1: Obtain fine quality dyes from the trading post. I advise buy orders, but if competition is tight you may be able to get some for very close to buy order price. I would advise getting 1000+ in order to avoid bad RNG. That's what I did to test this method.
Step 1B: Wait a few hours for your buy orders to get fulfilled. I had to wait a good three hours for all 1000 orders to be filled, and after announcing this it may take longer.
Step 2: Combine your fine dyes to get masterwork quality dyes. Be careful not to combine the masterwork ones!
Step 3: Sell your masterwork dyes for one copper less than the lowest sell order (not buy order!!!).
TIP #1: There are many, many dyes. I went through each and every single dye and placed a buy order for at least 40 on each one. Be careful though, many are repeated for whatever reason so you want to avoid repurchasing the same ones over and over again. This will take you a good 15-20 minutes, so don't rush it.
TIP #2: Know that the market is going to be weird after I post this, just like every other market I talk about in public. Don't be afraid to try it again in the coming weeks when things settle down.
TIP #3: This will effect masterwork quality dye markets, which could in turn make combining masterwork dyes profitable. It's a possibility, so watch out for it! At the very least you could buy them now and flip later. You may also want to hold on to whatever dyes you create this week and sell them later after competition gets lighter due to this strategy going public.