The most important advice that I would give to an e-commerce startup is you must have a product or service people want to buy. It also helps if they need to purchase the product, if not often, at least more than once or twice. Think things like toilet paper, or income tax filing as opposed to items people only purchase once or twice in their lifetimes. Once you have a great product, you must market, market, market. And you need an e-commerce site that works well with all the various mobile devices that exist today.
You must make the checkout process easy and that includes the ability for your customers to easily edit their shopping cart, see shipping charges and security information, and how to contact customer support. You must give your customers the option to order as a guest. You must also have great quality photographs, and a clear and easy to follow return policy.