Have you ever noticed how, if you are so inclined (I hear the pages make great rolling paper), you can acquire a bible for free? Go to any church, hotel, or come accross any wandering missionairies and you can leave with bible in hand free of charge (other than maybe your dignity, self respect, and/or hours of having to listen to the "truth"). Yes, there are various specialty bibles you do actually have to pay for, but it's the same words (WARNING: Translations may vary from minor to complete remodel) only wrapped in a fancier binding.
Based on that I ask this: How can the bible be the best selling book of all time if it's given out for free? Hell, even before I truly considered myself athiest I thought that statement was full of crap! This sentiment brought me to a second question: How are sales of books tallied? Mind you, I suck at research otherwise I would have already answered that question myself. My own conclusion is that sales from manufacturers to churches are being included in final figures contributing to the bible's lofty title.
If that is true, the title of 'Best Selling Book' is a farce and/or misinterpreted. It would mean that any organization or business buying any book from the manufacturer should be counted. That would then make the best selling book also the most printed book as whichever book printed the most copies automatically sold the most copies to organizations and retailers.
Thus, I have thought of an arguement point if a christian brings the fact that the bible is the best selling book of all time to the table. Simply because you and your church has bought hundreds of bibles per head to hopefully dole out to prospective converts (or those in need of cigarette paper), does not mean it is a best selling book.
Furthermore, it is not the world's best selling book, A Tale of Two Cities is.
I've had to buy 10+ Bibles because every time a friend brings one over my cat throw up on it. :p (Seriously. We call her the Atheist cat)
I think another part of it is how old is the Bible. It's one of the oldest books out there, so there's a matter of time. Sure, Harry Potter was very popular, but it's only been around for a short period of time. Also, I would argue the different versions are relevant too. I had a friend who had no less than six different Bibles. She had the teen Bible, the woman's Bible, etc. So that helps to increase the numbers dramatically, not to mention the people who want to buy the different interpretations to compare so they can know the "accurate" answers.