My observation has been that there's a strong correlation between "buy my ebook!" in a website sidebar and someone ticking boxes on some scumbag marketing process which they probably learned from someone using scumbag marketing processes to sell their ebook and lessons in the sidebar of their website about how to get rich quick selling ebooks and lessons. Apparently it works, but I'd prefer to avoid the whole mess, aside from using it as a strong signal to Beware everything else on the site.
Everything is a skill. Writing is a skill. Marketing is a skill. Some people will pay to read a book about the skill. Said writers would like to make money from writing said books.
I would think it would be fairly known if someone's books are snake oil. Just my opinion, though. Scumbag seems like a harsh judgement.
