Everything you wanted to know about building an eCommerce website and a lot you didn't! Here you'll find an expert analysis for why you should choose Shopify over Magento or why Lemonstand makes a better platform for new startups that are going places.
Bandwidth overage charges are a bad thing, and I advise you to steer clear of any platform that imposes limits on the amount of traffic you can receive to your website or the number of images you can store. Hosting is such a cheap commodity these days that there is absolutely no reason for it apart from nickel and diming you.
Also, you'll find some great tools and resources like my guide to finding royalty free images for your eCommerce store and a comprehensive guide to merchant accounts, payment gateways and eCommerce transaction charges.