1. Does it suck? Did you hire an actual editor, and take your editor's recommendations seriously? Even if you decide to fix something in a different way than the editor suggested, don't ignore an editor who says parts of your book don't work.
2. Does the cover look like you put it together yourself? You have maybe three seconds to catch a potential reader's attention as that reader scrolls past your book's thumbnail on Amazon. Does your cover look clean and professionally designed? If your cover looks like amateur hour, readers are going to guess that what's inside will be, too.
3. Do you have any reviews? How do you keep the attention of the potential readers who's paused to look at the thumbnail of your book? You need a credible number of reviews (my own opinion is that ten is a minimum to establish credibility), even if they're aren't all perfect fives. Get your book in front of as many people as possible and ask for reviews.
4. Is your book description boring? One paragraph: ten sentences or less. Set up the hero, the villain, and the conflict, and end with a teaser. Do not retell the plot. Read descriptions of best-selling work in your genre, and imitate what they do.
5. Are the first three paragraphs of your book any good? Go ahead, front-load the exposition, tell your writers' group that your story builds slowly, tell yourself that readers don't "get" you. But those excuses won't make a reader want to buy your book. Whether it's in an ebook preview or a bookstore flip-through, most readers will glance through the first few paragraphs to judge whether they want to buy a book. Those had better be the best three paragraphs in your book.
6. Do you have another book? The best way to sell a book is to have more than one. A satisfied reader will come back to find your other books, turning one sale into more.
In the end, the best way to sell books is to write more. Practice your craft, read other books in your genre, talk to other successful writers, read blogs like terribleminds and Fantasy Author's Handbook. Write more. Writewritewritewritewrite. And get better.