How do you do it?
Oh if you mean tricks in this game it’s mostly about money laundering, but it isn’t really considered cheating though. Just buy merchandises with pirate coins and sell them repeatedly and massively as you pleased in one single time. In short, you doing QE like the US gov. As for quantity, a few millions are enough. That process can be performed at the very moment you start issuing pirate’s coins. It doesnt have to be done at the early stage of the game but I thought you meant cheating on someone so, it’s not really that case after all.
You have to constantly remember what you buy with what price then when you travel to others island you can checks the shop to sell it. Do not relies solely on the stocks increase and decrease %, because the original price change first then the % affect the price once again.
I know this cause I noted down all original price of all items on each island (when the shop first opened up to you). And the price didn't match even when you took out the % increase and decrease.