We can break this down, if you like. Anything that you can applied the word "cheating" to is some kind of contest. Contests are designed to compare two or more individuals on some scale. For the scale "how good is a writer" we want to measure "how many people would enjoy reading my book(s)?"
As with most scales, the moment you apply points, you start diverging from the pure scale. Money is points. The first thing that disrupts this measurement is advertisement. When you advertise, you multiply the "good writer" scale by "how many people can I get my book in front of". Since that isn't what you are actually trying to measure, advertising is a form of socially acceptable cheating. Even worse, it's a form of cheating that completely overwhelms the original scale in the short term.
Purchasing a few copies of your own book is also cheating, but you could also call it advertising.