Amazon is the place where most sales are made (by self-published authors and traditional alike). So, yeah, you have to make sure you are at least there. When self-publishing Amazon has two models. Select – which is exclusive (meaning you can ONLY have your titles on their site) and non-exclusive where you can also sell on sites like B&N nook, Google Play, Apple’s Ibooks, and so on. You are paid the same royalty rate whether you are in Select or not.
Being in Select is not something you have to commit to for the whole time your book exists. You “enroll” for at least 90 days and can go in and out of the program in those 90-day increments. Being in Select provide some added perks including:
Income through the KU program (Kindle Unlimited) - the Netflix of books - where people pay a single fee and can read as many books a month as they want.
Being able to put your book free for 5 days in 90.
Doing “count down sales”
There probably are others, but those are the big ones. Some indies see 50% of their income coming through KU so many indie authors choose to go “exclusive.” For me, I keep a “wide” distribution (for added sales from other platforms, and because I don’t like limiting the choices of my readers to “just Amazon”
The amount of work an author does to promote their book is the same regardless of which route you go. Traditional publishers don’t provide much in the way of marketing to titles that get a standard advance ($5,000 - $10,000) and for books they acquire for six-figures, the marketing support lasts only a few weeks. Only the author is 100% focused on their books. The marketing department is VERY over worked and new books come out each week, so if you are fortunate enough to get marketing support it will be short-lived and not enough to really move the needle without the author doing the heavy lifting.