This is a really easy way to make some quick and easy gold in Runescape. What you want to do is head to a area with a lot of Cows. Cows are easy to kill and they drop cowhides that can fetch up to 300gp each and they are a very overlooked item. Either kill the Cows yourself and loot the Cowhide or if there are someone already there killing the Cows, just loot the Cowhide as most players will just leave it on the ground.
Since each Cowhide is worth 250-300gp you can easily make 100-200k an hour doing this and it is all F2P!