Adding a logo to an Amazon storefront creates more than just a refined and professional look for a sellers storefront, it is also rumored that a logo may help boost rankings for products that you are selling. This added advantage is especially important when you and your competition for the sale of a particular product are on similar foundations. That is you both have similar selling histories, feedback, etc.
It is vital for sellers on any online marketplace to set themselves apart from "the rest of the pack"; but this rule is especially applicable for those selling on Amazon.com. Having a good looking logo displayed on your storefront is a powerful tool in accomplishing that goal.