I believe this applies when you have constant traffic already coming into your website. A lot of people (moving from small brick & mortar store) to online E-commerce model, seem to believe that having a website would solve all their problem and traffic comes flooding in and sale going through the roof. I was one of them a couple of years ago until I wised up.
For those people starting your own website, the first thing to keep in mind -
Remember that creating a website is Step 01 of a long drawn success process. It is the most basic step and as much as I would love to say, you would start selling immediately, the probability of it happening is slim to none. A website merely ensures you have a web presence.
The logical progression to this is the next steps of marketing. Now that you are world wide web, let people know that you are on it. This is the painful process and consumes every bit of time, energy and effort you might be willing to offer.
Ideally SEO (Search Engine Optimization) which is a combination of creating exceptional keyword rich useful and related content, link building, forum posting, 301 redirects and so on. SEO is a medium to long term strategy to ensure your site is successful and it ranks in search engines. It has always been said that investing in good quality content would be appreciated by Google and other search engines thus bringing in more visitors to your site over time.
While you invest in SEO and Social Media (this will take quite a bit of time to yield returns) and it will keep burning your pockets, it is important to set aside funds for Google Adwords or other Paid Search program with them (PPC - Pay Per click). Essentially this means, you pay google/bing/yahoo when someone clicks on your advertisement. Popularly this is called "Google Adwords" (google being the largest of your traffic source).
If you do not have the money to invest in these (which could be well over $1000-$10000) per month, then my suggestion is to start a simple website with a check out cart and invest in Long Term SEO. In the meantime, use online marketplaces like ebay, amazon and if you have large volume products which can be sold at huge discounts then daily deal sites like groupon or livingsocial etc. This will ensure you have some revenues coming in, to support the significant burn rate through advertising. Good luck and feel free to reach out if you have questions. Thoughts welcome!