I am looking for customizable sample source code to build a simple website with
ability for users to register, logon and purchase items using paypal
or credit cards. The site will run on a linux box and I do not want to
store credit card information.

Testdrive CMS portals listed on the left bar of this site 
http://www.opensourcecms.com/index.php

Most poular is Mambo, I recommend Drupal. It has a PayPal component.
See more on that here:
http://answers.google.com/answers/threadview?id=607249

It is not the only way, but it is simplest. You just leave out 
all the modules you don't need. The core which remains is not much more
then user management.

Other ways  you will find under 

SEARCH TERMS: 
authentication apache
authentication
PHP script, user management, IPN, 

For example:
http://www.hotscripts.com/PHP/Scripts_and_Programs/User_Management/index.html
http://www.hotscripts.com/Detailed/18371.html

Some ISPs offer automatic installation of such scripts.

Hedgie