At the chapter 6 you may have problems with sandbox paypal service.
Paypal does some heavy tests.
Instead of the sandbox service of paypal you can use the beta-sandbox service of paypal. 

1) Use https://beta-developer.paypal.com instead of https://developer.paypal.com
2) Use http://www.beta-sandbox.paypal.com instead of  http://www.sandbox.paypal.com
3) Make the seller/buyer accounts there (at the beta-sandbox)
4) Go to \PayPal\Code and rename the pdthtml.php to pdthtml.old.php
5) Copy and paste the given pdthtml.php to \PayPal\Code\pdthtml.php

Sorry for the inconvenience.
