I'm ont sure what you mean by a "regular menu skin", but in the post entitled "CSS", I give a complete solution as well as the code required to skin the categories module. You could skin it to look like your regular menu colors.

You could do that in DNN.

Here's how.

Create pages using DNN's own menu, but instead of displaying a page, it goes to the URL of your BuyNow page, filtered by CategoryID.

In other words, if you had a menu item for "Mixers", in DNN you would have that page redirect to a URL that is http://yoursite.com/BuyNow.aspx?CategoryID=10 (where the 10 is the category for the mixers), and then don't use the Categories module.