An important part of SEO is the use of keywords in the headline in your website, and in your website content. Search engine robots look for high frequency words on your website. So if you sell books on space travel, keywords in your headlines and text might be: space travel, travel to the moon, interplanetary travel, space shuttle, satellites etc. The search engine robots look for the frequency these words are used in the headline, and in the text on your website. A high level of repetition of these words would indicate to the robots, that this is a site with a lot of information about space travel, so when anyone types in these keywords in a search, your site may be highly rated.
Unfortunately, high density repetition of keywords, while attracting the attention of robots, can be a real turn-off for humans. People make a decision in seconds on whether they should continue to browse at a website or click away. And the headline is the first thing that visitor sees and is the primary reason a person will stay a little longer. A compelling headline is usually inconsistent with a headline stacked with keywords. Copy with lots of keywords will also be less compelling.
So an overemphasis on SEO can actually reduce sales. Online sales consist on two basic components. Firstly getting traffic to your site. SEO can be important in this initial step. And secondly getting your visitor to take some action towards creating a sale (eg contacting you or actually buying something online). SEO has no role to play in this second step, and may well be counterproductive. This is particularly so if SEO has only a minor part to play in your traffic generation (compared with, say pay-per-click advertising).
Making you website pay requires the consideration of multiple strategies. For a website providing a product or service in a niche with little competition, it can be important, but for most online businesses, it often only provides an advantage at the margins.