I discovered that WordPress SEO and WordPress eCommerce plugins do not work together. I recently switched my eCommerce site from Platinum SEO to WordPress SEO. Unfortunately, it disabled all of the eCommerce capabilities of my site. The product page was empty, there were no "Buy" buttons if I went directly to a product. It may help me rank better, but I couldn't sell anything. Deactivating the WordPress SEO plugin resolved the conflict and the eCommerce functions returned.
I like this SEO plugin and hope that it will someday be compatible with my WPEC site.