For this guide you are going to want to head down to Port Sarim, it’s pretty easy to spot on the map. When you have traveled there, find the fishing shop. In the Fishing Shop you can buy Harpoons for 45g each. Harpoons on the Grand Exchange though is worth 170g so you can make a lot of money from buying these, you will make about 125 gold from each harpoon you buy. So fill up your inventory with Harpoons and head down to the deposit box on the docks and send all the Harpoons to the bank. Head back and get more Harpoons and rinse and repeat. This method of making money is so easy and you can really make a lot.