A resale certificate is a tax form that you can provide to your supplier in order to purchase inventory tax-free (assuming you are reselling the inventory to customers, of course!). This means that if you have the proper documentation, you can avoid paying sales tax on your purchases which can save you a good chunk of change. 

Before you get started, you should note that many states will allow you to use an out-of-state resale certificate to complete their documentation. But there are 12 states (plus Washington, D.C.) that require that you register for a sales tax permit in their state to get a resale certificate.