Traditionally, a bestseller was a book that was selling lots of copies in many bookstores.
Things got complicated with ebooks.
Every ebook has an Amazon best-sellers rank. Number 1 is the current best-selling ebook on Amazon Kindle and number 5,000,000 or something is the worst-selling book. Amazon updates their sales rankings every hour based on an algorithm. It's harder to manipulate these overall rankings, although I assume it could be done because it's a much more limited universe than a lot of bookstores.
But Amazon has created numerous subcategories of bestsellers. My latest book, The Lonely Dead, was ranked an Amazon bestseller in various subcategories numerous times. The rankings weren't manipulated, but it doesn't take many sales to make an ebook number one in a limited category (Children's Illness??) for one day or even one hour.
This week, I was leafing through Oprah magazine and saw an ad for a company that variously called itself a marketing company or a publisher. "With the AUTHORity Package, it's all done for you, and we can turn around a finished product-from interview to bestseller-in as little as 30 days. Our process overall is what I call a modern approach to publishing; we function in a manner that's considerably different-and less formal- than the traditional publishing houses....Every book has been a best-seller. Our success rate is 100 percent."
When I looked up some of the books featured in the ad, some had rankings around 4,000,000. So they clearly weren't really bestsellers for any length of time. I would assume they were "bestsellers" for a day or two.
How much does all this "success" cost? There's a stripped down program that involves three webinars and you doing all the work yourself. And that costs $1800! I can only imagine what the full program costs. When someone wants you to pay a lot of money for something, be skeptical and do lots of research.
In traditional publishing, money flows to the author. Personally, I like traditional publishing. I have experts who edit my book, design the interior, design the cover, market it, get it into libraries and bookstores, sell foreign rights, etc.
If you want to self publish, you should control things yourself, not pay someone thousands of dollars to do a (not very good, judging by the clip art covers I saw in the ad) for you.
Here are some ideas on how to do it yourself: https://www.janefriedman.com/self-publish-your-book/