In one of my Drupal based shopping cart site powered by Ubercart, the products were customizable(ie, end users had the permission to edit the product). So in order not to overwrite the existing product and to save the changes made by the end user, I had to create a new node with all the values of the product. So I created a new content type and related this to the product content type and saved the node as a new one. If you want to know how to convert a custom content type in a Drupal site into an Ubercart product, read on.