Even when you're the author or agent, it's not as easy to get accurate numbers as you'd hope.
With my first book, I knew how many books were printed (first and second printings) and I knew when the book was "unavailable in the warehouse," (as in virtually sold out) so you'd expect that the sales numbers would have been in the neighborhood of the two printings combined, but my royalty reports left 5,000 or so unaccounted for. Even with my agent's assistance, we never really did get straight numbers.
So, if the people directly involved can't get good numbers, I'd say it's fairly impossible for anyone to find out about another's books. In the case of best sellers, publishers will brag on the numbers, but for mediocre sales, they probably won't publicize those.
I wish it were otherwise though; there are a few books I'd like to have that kind of information about.