You can opt to sell your own items or you can choose to use the Fulfilment by Amazon service.
For the most part I use the Fulfilment by Amazon service because customers can use their Amazon Prime accounts to have goods delivered with FREE two Day Shipping and non Amazon Prime customers who spend $25 at the same time as they purchase an item from my store can opt for Free Supersaver shipping.
To get Free Supersaver shipping customers do not need to spend $25 at my storefront. All purchases from my store count towards the $25 total and purchases can be from several amazon store holders or Amazon.com itself.
My stor is also registered with Amazon to sell Internationally, so my sales are not just restricted to the U.S. but can be shipped to the rest of the world too. Raising my customer potential from about 300 million to over 6 Billion people.
Take a look at my storefront.storefront