In the latest version of the cart 3.96 there was a change in the category module so you will need to go into the category module settings and select the buynow page name again and save your settings and it should work for you. We updated the www7 test portal so you can see it working.
Here is a link to a page with the category module on a page without the buynow module:
http://www7.smith-consulting.com/
Here is a link to the page with the buynow module on it:
http://www7.smith-consulting.com/Cart.aspx