What sells a book is good writing. A storyline that captures their imagination, characters readers care about – heroes they want to root for and villains they understand. (Even the villain wants to be the hero of his own story.)
It’s really that simple.
Reviews can help, star ratings can help, but as you can see from the Bookbub graphic here, plot, price, and quality come before ratings and reviews. More importantly, though, not just any reviews, but honest, organic ones – in other words, not family/friends but from other readers. Readers are more likely to buy because a family or friend recommended it, but not a review from yours. (With thanks to Bookbub, see the link attached to the graphic for more information.)
Word of mouth is the best marketing.
Also, see this – https://killzoneblog.com/2018/07/it-helps-if-you-can-write.html
James Scott Bell knows whereof he speaks.
Reviews and stars help, but nothing beats a well-written book.
Update and addendum – Another voice about reviews from our friends at ALLi