I haven’t played in a while so I’m not sure if anything changed but basically:
1. Travelling between different islands costs food so always make sure that you have enough food in your inventory.
2. Prices of each item will change everytime you visit an island. This means a watch can cost 500 when you bought it but can be sold at 2000 on a different island.
3. With these in mind, you should stack up on food and buy cheap items on your current island. Travel to a different island to check if the item’s selling price is higher than the price you bought it for. If yes, sell everything you bought.
4. After selling everything, buy cheap items again and try selling it on another island. Repeat this cycle until you finally have a lot of money.