If only it was that simple! Last time I checked, there were several solutions to integrating buy now buttons with google analytics conversion tracking. Each one 'almost' works, but fails to track some percentage of conversions. The net is littered with posts from people trying to solve the problem.

The 'correct' solution, is that paypal can callback to a URL at your site with details of any completed transactions. You need to run a script at that URL, that will put HTTP request through to googles tracking servers, pretending to be the customer. The trick is matching up the information received back from paypal, with the customers browsing the site and spoofing the request to googles tracking servers.

I believe a consultant wrote a blog somewhere explaining how he implemented it for a client. When I looked it, it would take about 3 or 4 days of effort to code up a solution.