rss javascript coding

We have a page which creates an RSS feed. We would like a page that allows a user to enter their own PID (for ebay). Then, we want a script to be created, which they can put on their site, which will send our RSS along with our PID 50% of the time, and their PID 50% of the time. A good example can be found here: [login to view URL] a clone of that would be ideal.