A High Rankings thread asks members what is a good non-open source, but commercial shopping cart software package that is search engine friendly. Three people in the thread tell the guy to build a custom solution, because that is the best option (of course I agree). One person recommended x-cart and an other recommended SecureNetShop.com . Also, mcanerin recommended that you use any shopping cart, but build the product and catalog pages from scratch so they are search friendly. All of these solutions are fine.
The High Rankings Forum has a whole forum devoted to Shopping Cart Discussion. I also have a basic article on how to construct your Search Engine Friendly E-Commerce Catalogs.
Forum discussion at High Rankings Forum.
