Suppose your Drupal projects handles products which can be purchased through online. And you are using Ubercart PayPal and Ubercart Payment modules to handle payments related to purchase. Either registerd or non registered user can buy the product. User can use paypal through two ways. "Can purchase through sandbox environment or Can purchase using credit card". So a tester can test the paypal functionality using both these ways. Here I am going to explain how can you test the purchasing functionality through Sandbox.