If you have a PayPal account and wish buyers to pay you via PayPal for items you sell in the market area, you first need to set your Market default settings:
Go to the market area and click the "My Market" tab.
Next, click the settings link below the tabs.
Once you set up your PayPal address, when you generate an invoice for an item you've sold in the market, the buyer will automatically see a "Pay with PayPal" button in their "Buy/Bid" screen. It's that easy.