I was in a bookstore recently and despite the pretty covers, books laid out at eye level, and being able to handle the books, nothing, but nothing, could entice me to buy a book. The next day I was in another bookstore and they had loads of books that interested me and I bought two, and had to restrain myself from buying more.
Yes, the covers had a part to play in catching my eye, but it was the subject matter that made me decide to buy.
The POD authors who are battling to get their books into bricks and mortar bookstores might want to ponder on this.
Actually, self-published authors would do best sticking to niches. Every man and his dog has published a book about nuclear meltdown, good versus evil, and end of the world scenarios. Fiction itself is not a particularly good field for POD; you are competing against millions and millions of other books and it is hard to make any headway. Why do so many 'so called' celebrities publish fiction? Because they can sell loads of books, based on their fame rather than the merit of the work.
Specialize, branch out and investigate every tiny niche and possibility and you might start making some sales.