I've run a sideline in art, various mediums, for years, and I've been self employed as a maths tutor, and I would have to disagree that the product advertises itself, however good it is. You have to think at least a bit like a businessman, and that means looking at how you'll market to people. It very definitely also means having a good product to start with (or at least one that will appeal to some people - 'good' is probably too strong a term for some things that sell really well).
I know what Tinkerdan probably means: If your product is solid you should get business via word of mouth, people impressed with your product will sell it to others for you. But even if that's your main marketing strategy you need to convince those first few to try, and that means advertising, even if it's highly targeted. My experience of social media was that the effectiveness depends on how well you're able to target your message, and how open your market is. For example I targeted local parents facebook groups and local facebook pages as a tutor - many were actively looking for tutors, others were open to letting me talk about what I did. Be upfront with what you're doing. I got clients that way, and got the ball rolling on word of mouth. But selling artwork has taught me that you need to build a social media following, a fanbase so to speak, because for that product you will get a lower percentage of people who come to look that will actually decide to buy (art is not so vital to most as their children's future). Sometimes sales have been of pieces I didn't expect to go at all, or came from entirely unexpected directions - spreading a wider net seems to be the way to go, though you still need to build a relationship with your prospective clients. I suspect selling a book, assuming it is well done, is much more like the latter unless the subject matter is something very pressing on a lot of people's minds.
On a related note... has anyone here tried using Patreon, or a similar site, to support their writing?