If all you say is that when someone buys your book, they get a book and you get money, that's off-topic. This is not, after all, a forum about "the things you can do that get Trevor money". Just because it is uncontroversially true isn't enough reason to go about saying it here.

Commercial links aren't rejected just because they are commercial links, but it is unlikely that a random commercial link would be relevant, and they certainly can be rejected for not being relevant.