Though my sales are miniscule, and I don't have anything on any list on amazon.com, I did just find out a novel under my pen name is #59 on the paranormal fantasy list and #99 on the romance list...in the UK.
So what does that mean? I can say I'm an Amazon best selling Author, an Amazon.co.uk best selling author, or nothing at all?
Good question, I've been wondering the exact same thing. A fantasy book I wrote with a friend cracked the top 100 of two separate lists during its free promo - one being Epic Fantasy, and the other being Classic Literature (because our story was a myth that goes back to the eight hundredth century, so essentially it's public domain).
It was pretty funny, surreal and WAY undeserving seeing my name in between Hamlet by Shakespeare and the Complete works of Dickens.
At the same time, I don't feel like I can rightfully call myself a bestselling Amazon author, because, well, for starters, I start a new 9 to 5 occupation the day after tomorrow which is indisputably going to be a normal shit job.
Anyways, I asked a question close to this a while back, and they told me the same thing - you CAN call yourself a bestselling author and not be entirely deceitful. This is a fairly new platform, and not everyone knows how to label categories specifically, even ranking, which most writers are obsessed with (myself included).