The most important thing when you are building an e-commerce website is the safety of your customers. Using secure HTTP (HTTPS) is a must, as this will encrypt the connection and thus protect the user's personal details (credit card, payment methods, etc.) from being looked upon by prying eyes.
Securing the back-end of your website is also a must, as there are many people out there who could easily exploit an e-commerce website if it was poorly built and did not have much protection against attacks such as SQLi, CSRF, manipulating GET variables (for example, http://domain.com/item?id=1&cost=100). There are some great resources out there that cover the vulnerabilities of any e-commerce website and what you should do to protect against them,
to name a few.
As for design features, make sure your items are easily accessible. When the user browses to the home page, they need to be instantly attracted to what your site has to offer. Show off some of your most expensive items, maybe even some recently added items. If you're very experienced in this sort of area, you could even add a listing of items suited to the viewers tastes (but pay attention to the EU cookies law). Good examples of this are common websites,
Another key design feature to consider is the ease of use in your site. Make sure the user doesn't get lost, and make it easy for them to go back to where they came from. Using breadcrumbs in pages are a great way to do this. For example, if you got to page "Bar" from a link on the "Foo" page from the home page, it would be something like 'Home > Foo > Bar' with hyperlinks on the 'Home' and 'Foo'. Don't overcrowd your webpages, the item the user is viewing should be clear and the first thing they see, not a bombardment of advertisements.
There are some major things to consider while making an e-commerce website, one of the main being legalities. Make it clear what you are selling, how much it costs, the delivery methods and costs, terms and conditions, refunds, etc. One of the last things you want is for your users to file a lawsuit against your website, trust me!
Experience in web development, research into e-commerce and user experience.