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Omnomberry Compote is probably the best healing buff food for pvp currently, and it certainly is popular at the moment on websites within the community. I've seen the recipes themselves selling rather quickly, so that might be an opportunity as well as straight up crafting Omnomberry Compote.
You can place buy orders on omnomberries and directly purchase the bags of sugar.
1 berry + 1 bag = omnomberry compote.
At the time of this post, you would make 10 copper per omnomberry compote sold.
Obviously, things are going to get tight now that I've posted this, so don't be surprised if things change for the short term. But rest assured, it will make you gold later on when the market self corrects and people get bored of this method (like everything I post on here). Right now, I expect the price of compotes to go down and the price of omnomberries to go up. Bags of sugar won't move too much, maybe 1 copper at most.
Lesson from this: if websites mention something being popular or "the best" for any aspect of the game, then go craft and sell for a profit.
But also learn from previous mistakes and tweak your crafting. Find ways to get items cheaper and also avoid over-posting to the point of having to cancel in order to sell fast enough.