I'd just popped in quick to check my Twitter before sleep, when I saw YA Twitter talking about all this. TL;DR: An unknown book hit #1 on the NYT YA Hardcover Bestseller List. People investigated how that came to be: small bulk-orders. Details at the link.
The thing about bestseller lists is that people like them because they think other people like them. e.g. Authors like making the list because it may make them more attractive to publishers/readers. Mass generalistion, I know. We've all heard rumours about bestseller list curation.
Really, the only true way you know how your book is selling is via your statements. Earn out your advance and gaining royalties after? You're doing well.
But that's not enough for some, because other people can't see your numbers. Which reminds me of that Family Guy cutaway about people writing in public, because there's no point writing if there's no one to watch you do it.
Anyway, as a reader I'm not more impressed by an NYT bestselling book than one that didn't make the list. Because I don't trust other people to like what I like. Other people have different standards. (When I'm feeling particularly bitchy, I think they have LOWER standards ;-) )
So how much attention do you pay to bestseller lists? Have you chosen to read something specifically because it made the list? Is the Bestselling Author rank prestigious and meaningful, or just a bit of bookwank?