Well, Ebay and Paypal both charge fees. You have the choice of how you want those charged, but basically you have an Ebay account and at the end of each month, you are required to pay Ebay for those fees--you can have Ebay automatically charge a credit card or your paypal account or you can manually send them payment each month.
You will get charged listing fees up front to put the item up for auction. Then when the item sells, Ebay will charge you more fees called Final Value Fees that are a percentage of your selling price. Then, if your buyer chooses to pay with paypal, paypal will automatically and instantly take a percentage of the payment you were given and then deposit the remaining money into your paypal account.