Well said oddz
While you can find stock Comparison Shopping Scripts, sites like TheFind, PriceGrabber, etc did not grow to their level of success by using generic features. Building the core isn't difficult but as you dive in to the CSE space you'll see that the complexity really lies in the collection, management and display of product data. The feeds going into one of these sites include millions of products which are updated daily or more.
Presumably you'll want to focus on a niche or new angle to stand out so whether you chose to build from the ground up or modify using an existing system's base, you should really plan to put some solid time into the customizations.