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Have you unlocked the "hidden" Sisyphelly scene? If not, you can refer to this patch note for more clues: https://nonohomocircle.itch.io/pirahus/devlog/675524/pirahus-build-17

It's in the main story, just follow Sigurd route and you'll get there eventually :D

1. First, go back to a save file where you have like >4k coins or something (the more the better) 
2. Then you need to hoard lots of goods (buy them whey they are sold at low prices- as indicated by the green arrows).
3. Next, travel to other islands and check their markets (sell mode), if your items can fetch a higher price (higher than their normal prices) then sell them. If not, you can either keep travelling between islands till you find a reasonable price (which might take tons of supplies), OR load your save file, buy the same goods again, and go for a different market (this way you dont have to buy more supplies) 
4. Repeat the process and profit.

Another thing, lets not touch the pirate coins rn, i think the exchange rate is buggy atm, hopefully it will be fixed in the next patch.

Spoilers: 

















You need to buy the coral in the gift shop 

Not sure if this is the fastest way but here's how I dit it:
Head West 10 times, then keep heading East (each time you encounter the monsters, use flare)

You can continue from Kotetsu part and at the end, give Leopold a call :D