In Dead Island for the PC, PS3, and Xbox 360, there are a couple easy and repeatable ways to get as much money as you’ll ever need.
Regular Method
You’ll notice fairly quickly that items in the game respawn. If you find an area that has several items that sell well, simply take them and fast travel to your nearest store to sell them. Go back, and the items will be there again to take and sell as many times as you would like.
Glitch Method
There are a couple different ways to duplicate items in Dead Island. The most efficient way however is to wait until you reach a checkpoint, and drop whatever items you want to duplicate onto the ground. Kill yourself, and when the respawn timer comes up, pause the game and reload the checkpoint. You’ll still have all of your items in your inventory, and the items will be on the ground as well.