On Page is an important task that must be done before performing Off Page task on it. On Page tells whether the website is designed as per the Google algorithms or not. Without correcting your On Page as per the algorithms of Google you can't rank it in different search engines quickly. Meta tags, Meta description, Alt tags, H1 to H7 tags, Sitemap, Social Media Integration etc are few of the crucial points that must be included within a website to get it the best rank in Google search engine easily.
Secondly, the Off Page of Ecommece site can be done by elaborating the details of it in different sites wherever you are posting. It won't take much time to boost the rank of an ecommerce site in Google, if you provide the exact info to them for which they are searching Internet daily.