From Wikipedia:
Well-known bestseller lists in the U.S. are published by Publishers Weekly, USA Today, The New York Times and The Washington Post. Most of these lists track book sales from national and independent bookstores, as well as sales from major internet retailers such as Amazon.com and Barnes & Noble
If the book I author appears in Publishers weekly, NYT, Washington Post or USA today as a bestselling book, before the close date, then this market resolves yes.
If all of these lists cease to exist, then resolves N/A. if any of them still operate, then they count.
@mods is it even possible for this question to resolve now that the creator is banned and as far as I can tell hadn't revealed his real name?
Maybe I'm not understanding this but apparently only 0.1% of published books are best sellers. Anyways, I like the ambition
fun fact, with enough money you can cheat your way onto these lists by buying truckloads of your own books, which is how a lot of the books get on the bestseller lists.
@MichaelWheatley If the co-authoring is a significant enough thing to make it so that both authors share the authorship, then it counts. Assistance to another for writing their book doesn't. Something like Good Omens counts:

@firstuserhere Thanks. I was afraid of another AI Letter situation so I'm trying ot work out all the angles.
@firstuserhere the longest thing I've ever written was 8 years ago, a 15k word long story centred around death and regret.
(Excluding any theses written)