Go to My Profile 》Shopping cart settings, there you will see Payment Methods where you can enter the Paypal email address which is associated with your Paypal account.
Next, you must select the enable tab to allow accepting payments through Paypal in your listings.
Please note that when creating a listing, the Paypal payment gateway will only appear in your listings as a shopping cart setting by selecting "Auction" or "Fixed" price format and not in "Ad".
Auction allows bidding and Fixed allows a one time fixed price set by the seller.

For example, a buyer will purchase your item by adding it to their cart and proceed through checkout by using the paypal payment gateway option you provide in your listing.
This transaction is all automatically done and you will receive an email notification of the completed transaction.

See images below for a step by step visual: