My favorite used bookstore in college was an unorganized mess. It was barely separated into genres, and not at all after that. I saw one person walk out in disgust, since they couldn't find what they were looking for.

I loved it. If books are sorted by author, I quickly lose interest in browsing. This way I was forced to basically spend hours looking at each book to see if it's something that struck my fancy. I totally loved that place.