Data and Image Transfers Using XML or other API

We have a growing list of clients who sell the same line of products. However no two of them sell exactly the same items. This process should have a simple solution - so don't let the fact that we are trying to be as exact as possible in our description of this project cloud your judgement. What We are looking for the easiest way to "Automate" our site building Process. We have our own Content Management System - that is proprietary to us. All of the work will need to mesh with our system. In layman's terms, we want to develop a "Master" web site that contains all of the possible Products our clients sell, and the associated images. Then, by selecting ONLY each product they want (almost like adding it to a cart using check boxes or something similar) we want to be able to grab all the associated data, and the images and transfer it to the new sites we are building. We will build the master site - and load all the products and images. What we are looking for is the "bridge" system between the Master Site, and the new sites. Database structures will be virtually the same, so perhaps a simple XML protocol would be the best solution. Please include your ideas for how to handle this. Please Please Please - DO NOT include a long resume, or list of sites you want us to look at, your bid will be ignored.