If you are not enrolled in kdp select, you can publish your books on as many other websites as you like. After a 6 month trial I decided to ditch kdp select as on places like Smashwords, you can publish your books to lots of different platforms at once. ibook store, kobo etc.
Anyway, I screwed up my pricing on Smashwords and accidentally started giving away my books for free. (In fact, always make sure you double check your pricing on SW after saving, as I think the system might be buggy). At the same time, though, I started selling more on Amazon but when I realized my mistake and started charging at SM, my sales on Amazon started to flatline again.
All that said, there is no secret to selling books successfully. I was lucky in that I wrote one book in a very niche area and it quickly got picked up by an influential blogger who promoted it without me even asking. Then I (because my book was about unsolved murders and suspicious deaths) I was promoted FOC by a surviving relative of a victim I mentioned in my book. That too, though, was completely by chance.
In fact, I have had a golden run of luck as far as marketing goes. However, I still only make $40 a month. The trick, in this case, is not to think you are going to get rich by self-publishing. You’re not. All you can do is try to slowly build up a portfolio of books which slowly gather some great reviews, before publishing more work and building on that success. My only problem is that I rarely have time to write now or focus on marketing my books anymore.
Anyway, good luck.