An effective SEO campaign can significantly increase organic traffic to your site, but it doesn't guarantee sales conversions. There are many external variables affecting conversions that are simply outside the control of your SEO expert, including demand, pricing and other market forces. That said, there are a few complementary practices that the SEO team recommends to all ecommerce clients: design and marketing.
All the SEO in the world cannot improve a shopper's reaction to a poorly designed website. The shopper should immediately "feel" your online store is legitimate and trustworthy. If they don't trust your site, they won't purchase a product nor will they come back to the store to browse in the future.
Likewise, SEO on its own does not make up a complete marketing strategy. Your online marketing strategy should also include complementary elements such as social media, PPC (pay per click) advertising, and shopping feeds. There's no exact recipe for success, but it's rare to find a successful company that doesn't actively market itself through multiple online channels.