What does it mean for a site to have a pr5 for the non www version and pr 0 for the www version since I'm looking to purchase a link on that page.
If you buy a link from it, you'll still be getting the benefit from the non www version. It happens because all the links point to the non www version, and the seller has not bothered to redirect one to the other. Ideally the seller should redirect his www version to the non www version.
It means the site has canonical issues which leads to duplicate content issues and split page rank/link juice. Google and the other engines index URLs, not web pages. They see http://site.com and http://www.site.com as separate web pages, even though the exact same content is rendered (the home page). They should pick one URL for their home page URL (this is called the canonical or preferred URL) and 301 redirect all other home page URLs to the canonical. The non-www is PR5 because most links to their home page use the non-www. They probably have a few links where they use the www URL. So if the non-www for instance has 1000 inbounds and the www has only 10 inbounds, you can end up with this situation. If they implemented 301 redirects from www to non-www to enforce the canonical non-www URL, then their non-www home page would get credit for 1010 links instead of just 1000 and the www URL would be dropped from Google's index.
If content of both www and nonwww is the same there`s not difference for Google. Remember some PR are not show on toolbar.
www and non www are both seen as 2 different sites to the engines, there fore that is why different pagerank.
Just to digress slightly, if you have a site that hasnt had its redirects setup correctly, then you can tell Google in 'google Webmaster Tools' that www and non www are the same thing. This could help overcome the site getting dropped and any duplicate content issues....