I buy used books if available on amazon. Book market will become more niche with people shifting their time more to online media.
I’ve been known to buy a book after I read it, if I really love it and want it to be part of my collection. I’ll also buy books of the authors I know. My Dad has a friend who is a successful author now. From the beginning he would go buy his new book the first day it was available. Pretty sure if I publish a book my Dad would go buy a copy first day it was available.