Neither of these things. I am certainly not arguing that Amazon is unethical. Only that it can be ethical for people to make different decisions about when to take advantage of someone else's mistake on a case-by-case basis, based on the circumstances involved, without being required to produce a bright-line test valid in all possible hypothetical situations, as you seemed to imply. I'm just of the opinion that it's important to accept nuance and common sense in ethics, even if it means that sometimes dilemmas will arise that cannot be resolved easily. I'm also rather convinced that no human moral system exists without such dilemmas anyways. But the existence of dilemmas does not mean that every case is a dilemma, and it can be ok to make different decisions in different actual cases without having an answer to every hypothetical case in between.