Dean Wesley Smith suggests here that what makes a book sell isn't so much promotion as the ability to write "a great book."
Well, to a certain extent that may be true. I know some terrific writers who aren't finding an audience, but on the whole I'd like to imagine that in the long run, cream rises. But I disagree with this statement: "And a bad or amateur story doesn’t sell no matter how much you Twitter and Facebook and blog tour about it. Sorry."
On the whole, I think this is usually the case. And yet I have seen the occasional indie book that really sucks, and yet is way up in the top 1000 on Amazon. Specifically, I've seen this in romances, which is the genre I'm most familiar with. But it may very well be true in other genres too. Sometimes poor-quality crap does in fact sell.
At this point the argument often morphs into, "Quality is subjective," or "Obviously that book has some appeal to readers, and it's the writer's storytelling ability that counts." Maybe. But quality isn't entirely subjective; I've seen some high-ranked books with appallingly amateurish writing and plotting. And sometimes a book sells well just because it happens to be riding the coattails of a successful trend, or even possibly because the writer has gotten a lot of friends and family to leave positive reviews, so that readers are temporarily tricked into lifting it up the charts.
I will grant you that in the latter case the book's popularity probably won't last long. I like to believe that in the long haul, good writing will tend to win out. Even so, I think it's a bit simplistic to posit that "bad or amateur" stories never sell, because sometimes they do make their way up the charts.