It's been a myth for as long as the Internet has been in existence. Get good rankings and your site will be successful. I hate to be the bearer of bad news, but that has never been the case. What you get when you achieve good search engine rankings is a lot of traffic. That *can* be a wonderful thing, but it by no means guarantees even one sale.
You see, search engine rankings are not responsible for making sales. They are responsible for getting visitors to click to your site. If you (or your search engine optimizer) have done your job, you'll have descriptions in your search returns (or PPC ads) that bring in qualified visitors. But still. even with qualified visitors, the engines are not in a position to make sales at your site. That's the job your copy and usability elements handle.
Top Security, Inc. faced this problem. They had good search engine rankings. They were listed on the first page of most of the major engines. They had the traffic. Sales, however, were not where they wanted them to be. What was the cause? One look told me the copy was the culprit. (Take a look at the original copy here: http://www.copywritingcourse.com/topsecurity-original.pdf.)