This is the default functionality when the payment type is set to paypal (version 7).
See CMSDesk->Ecommerce->Configuration->Payment Types and Site Manager -> Settings -> E-commerce -> Payment Gateways -> PayPal
See the demo ecommerce site, which implements PayPal gateway.
and http://devnet.kentico.com/docs/ecommerceguide/index.html?paypal.htm for more detail setting it up.
You can also set up sandbox buyers and sellers with PayPal to test with; with a developer PayPal account.