Most of us are not "Brands" and our potential customers do not know us. Therefore, an iron clad guarantee helps take the risk out of the buying decision.

Also as Abhishek mentioned, providing an easy and clear method of contact also helps, as long as you respond to questions and refund requests quickly. By using an autoresponder to confirm reciept of the request with a time that you will respond (managing expectation) is very important. In some cases you might even be able to win back the customer.