You must make your site as easy to use as possible. Potential customers won't waste their time trying to work out how to use your site. If it's not immediately obvious how they can get what they want, they won't do it. Not only should the UI (User Interface) be optimized, but the site itself should be designed to make it as easy as possible for visitors to convert to customers - i.e. it should be optimized for conversions.
It is important to regularly analyze how your online business performs and look for areas of improvement. In particular, you will need to implement some form of traffic analytics for your website. Analytics (I recommend the free Google analytics) allows you to see where traffic has come from and what it does on your site. This helps you position your marketing efforts and improve on under-performing areas, while driving home your advantage in areas you are succeeding.