Personally, I buy physically when I am in Japan - mainly at bookoff.
Digitally, I am mostly using bookwalker because of the coins/discounts and very rarely amazon.
Oh and selling: I don’t. I just give them away, either to ppl who are in Vienna or to one of the multiple open libraries around me. I also plan on contacting our local Japanese international school to see, if they might want them. But haven’t gotten around to it.
I keep a shelf on bookmeter.
