Dean Wesley Smith notes that if a book sells well, perhaps that's because it's a good book.
Of course, we've all seen books we don't much care for sell well, but that's just further evidence that readers can't agree on what constitutes a good book. I would argue that once a book reaches a certain level of sales, there are people reading it just to see what all the fuss is about. But getting it to that level of sales is a group of readers who really, really like that book.
That's also why I think it's important to have a healthy dose of skepticism about one-size-fits-all marketing programs. Books are not all the same.