The product listing page of the cart allows you to filter products by category by passing the category id in the querystring, for example:
http://siteurl.com/default.aspx?categ....
When the category id is passed in the querystring the products are filtered by category.You can use an html module and add category images or just links and create hyperlinks that link to the page with your buynow module on it and pass the categoryid in the querystring to filter the products by category.
Thanks,
Christian