Publisher is best used for small, simple static websites in my opinion, but
you can use Paypal and such features. If you are only selling a couple
things then it would probably be fine. The bigger the site and the more SKUs
the more difficult it will be to manage your site.
You use the insert html code fragment tool to insert the paypal buttons and
code snippets provided by the service. Go to the Paypal site to find the
instructions for how to build the code snippets. Here are links to three
Publisher built sites that have incorporated ecommerce functionality. You
might study their code source and the site structure to get an idea of how
to do this
http://blissdesigns.net/index_files/pearlnecklace.htm
http://www.timesaversforteachers.com/index_page0006.htm
http://www.dragonflymagic.com.au/index_files/leadlight.htm
Also if you are just starting to plan or build your site you might consider
Serif WebPlus. I believe it has more built-in capability for paypal and
using the software is similar to using Publisher.
DavidF