This is how selling books on Amazon works as an indie, after you’ve exhausted your own lovely readership.
Sales rank is like shelf space. The higher your sales rank, the more likely you are to be seen by random browsers. (This is also influenced by # of reviews, especially when it comes to browse methods other than category-wandering, like ‘also bought’ and straight up search results.)
Sales rank is obviously influenced by sales. So you need to move your book up in the sales rank so more people see it. You need to do this by selling. Or by doing giveaways that you convert to a minimal price point on the second day. And you have to promote the sales and the giveaways, so you go to various deal-promo lists and you hope to get a lucky feature or else you pay them (or you get lucky and THEN you pay them) for that feature.
And you hope like hell some of the taste-setters maybe pick up your book someday. One of the problems there is that the taste-setters are pretty courted and petted and busy, and they often have an aversion to reading books they haven’t heard other people talking about warmly. Kind of a catch-22 there, where you also have to end up lucky. But you keep trying because word of mouth matters and some mouths have more reach than others.
Meanwhile you’re of course working on the next books, which, especially if they’re sequels, provide their own boost to sales. I think they’re the longest-term thing to be accomplished, both in terms of time spent and benefit gained. But ask me about that again in a year or so!
It’s definitely a job, with expenses. Not anywhere near a full-time job, not for me and my shelf and my approach. But I can understand why a micro press without somebody armed with a budget and fully dedicated to marketing would eventually flounder. And unless you have lottery-winning luck or an existing platform with thousands of solid social media contacts, simply writing a book and putting it out there is going to result in six sales and depression.