It's just like any other business; in many ways the site is the least important aspect. If you use common sense regarding design, on-page SEO, content, etc., that's a done deal. Where most people fall down is poor promotion, failing to do the work required to get backlinks, and so on. If you rented a real storefront on some street, stocked it with product, then just sat there, you'd lose your shirt. Promotion without being spammy about it is the crux.