While doing the testing of our paypal smart buttons integration we use test client ID and do the test payments and for that we need to create test business/seller account and individual/buyer account where we get the test username and password using which we can login to our sandbox account to verify the payment received in our business account and sent from personal account. Once we done with the testing then we modify the test sandbox client ID with the live client ID. You will get the complete instructions in the video that how you can create the client ID from your developer paypal account.