Jeff Muendel writes in this article featured on Practical eCommerce:

Traffic down? Conversions starting to wane? It could be any number of SEO issues. Finding free online tools to hone search optimization for an ecommerce site can be tough. There are a lot of them out there, often promotional in nature, and they offer varying degrees of features and reliability. Some spit out data that is simply erroneous and applying this sort of information to website design can be useless at best and deadly, in Internet terms, of course, at worst.