In order to sell an iPhone 5S or any other item on eBay, you must first set up an eBay seller account. In order to set up an account, you must either have a PayPal account, checking account or credit card to pay eBay your seller fees.
Once your account is set up, you must create a listing for the phone. You can adjust the starting price, auction duration and shipping method. You can also choose a Buy it Now listing in which you can set a flat price for your phone. After a buyer pays for the phone, you then ship the phone.