if you have a dynamic website then seo need to be done dynamically. if static thats a manual process. thats on-page seo. then there is off-page seo of course.
Generally, the same way you SEO any web site. The main difference IMO is that ecommerce sites typically have lots of URL canonicalization problems leading to split page rank/link juice and duplicate content because: 1) URLs are typically dynamic and riddled with query string parameters 2) a product page may appear in multiple categories (TVs, HDTVs, Electonics, Sony, etc.) Query string parameters cause major issues on any site... but especially on ecommerce sites where their effects get magnified. Simply changing the order of the parameters from ?catid=123&prodid=456 to ?prodid=456&catid=123 generates a new URL even though it renders the same content. On many ecommerce sites, the URLs usually include lots of additional parameters to control sort order, track which path through the site the user clicked to find the page so the breadcrumb can be altered, etc. My recommendations are use search engine friendly URLs. Let each product page have a single URL... a single place it lives... a single breadcrumbe. Regardless, of the path through the site taken to get to a product page, once they click on the link to the product take the user to its "real" home... Use anything other than query string parameters to control things like sorts... use cookies, JavaScript, whatever. Just avoid query string parameters at all costs.