Well it still wouldn't really be an "official" guide, because Billboard only have accurate numbers for United States. Their guess is as good as many others.
That's what I'm also thinking and it's just impossible Thriller's sales doubled after MJ's death.
Why is it so hard to believe people use to actually buy records then? And, you all are forgetting the fact, a lot of those albums are still being bought.
It doesn't even matter how many copies Thriller has sold. The way they count it is messy and imprecise, but it's obviously the best-selling album of all time.