Marketing is not Sales (or selling) and same on the other side... Sales (Selling) is NOT marketing. Are they interchangeable? I think not... Do you need one to have the other.. yes! You need marketing in order to sell no matter what industry you are in. Marketing a product, service or even real estate doesn't mean it will 'just sell'. You need to use marketing, promotions, skills to then SELL 'it'.
I read this piece below that I think will help define for most of us the difference.
courtesy of Laura Lake, About.com Guide
What is the difference between marketing and sales?
"Marketing is everything that you do to reach and persuade prospects. The sales process is everything that you do to close the sale and get a signed agreement or contract. Both are necessities to the success of a business. You cannot do without either process. By strategically combining both efforts you will experience a successful amount of business growth. However, by the same token if the efforts are unbalanced it can detour your growth."
to read more from this article: http://marketing.about.com/cs/advertising/a/mrktingvssales.htm
As you can see, we need both but always remember there is a difference....marketing is not sales!