I'm currently in the process of building an ecommerce site. There's a test site up at http://www.dearjackalope.com/wesupplydirect.
In the header is a link to my RSS feed at http://www.dearjackalope.com/wesupplydirect/feed, which returns a Page Not Found error. The comments and homepage comments feeds are both functioning as anticipated so I suspect it's not a plugin clash issue although I guess it may still be.
I've reset permalinks (including with my htaccess file permissions set to 777 in case writing to that was the issue), to no avail. The default permalink RSS feed URL doesn't work either anyway.
I've got no actual visible links to the feed on any pages, but the big problem is that I've got Better WP Security installed which locks out IPs which access 404 pages too many times - unfortunately it considers the broken feed link in the head to be a 404 and locks out every IP which loads more than 3 pages on the site, so I really do have to fix this ASAP.