Definitely agree, SEO is a great trafic source, but nowadays it's better to diversify trafic source to ensure that your business wont be penalise if google or bing improve their algo...If i can add something to this answer, that would be to add seo to a social marketing strategy, they defenetly work together and as seo expert for a lot of ecommerce websites in France, i can confirm you that have a presence on facebook, twitter, google+, linkedin,etc...adds value to your seo efforts

Great answer. I've personnaly observed that a lot of new ecommerce websites organise a contest to attract people on the new website. This is a great mkt tool because the brand build awareness before the real website goes online. in order to do that you should use your emails contacts or facebook friends if you're a small player or if you have more to invest you can rent some emails address on dedicated platform