What draws your attention to a book on a shelf? Is it the colors? The font? The cover illustration? The blurb? The author bio? Awards the author/story have won? Reviews? Or a combination of any or all of these?
If your front cover (or the spine) catches a potential buyer's eye, you have about 3 seconds--yeah, that's all--before he/she turns the book over and looks at the back cover. If you lose him/her in that first 3 seconds, your chances of making a sale are pretty slim. But don't give up all hope yet; you still have a chance if the material on the back cover is intriguing enough to pique the person's interest.
I'm sure you've heard the expressions, "Different strokes for different folks," and "You can't please all of the people all of the time." Truer words were never spoken. That's why it's so important to make sure your book cover is attractive to a wide variety of people--if you want to attract their attention to it so you can make sales.
Earlier this year we covered the art of writing an author bio. If your potential reader can identify with you, you have a much better chance to make the sale. Your blurb (which we'll cover in a couple of weeks) is of utmost importance in snagging a buyer. At any rate, you have to make sure a person cannot possibly leave the bookstore without your book.
Each printing company has a list of requirements which they call "Guidelines" for setting up your cover. They are not usually in a common vernacular, so it's much easier if you hire a graphic artist to do the setup/layout for you. I have seen far too many really good authors place their books on the market with covers that appear amateurish. And quite frankly, people avoid those books. After all, if the cover doesn't appear professional, how can the interior of the book be any better?