1. Harm to others: Buying/selling X causes wrongful harm to third parties.
2. Harm to self: Buying/selling X causes wrongful harm to the participants themselves.
3. Exploitation: Buying/selling X involves taking pernicious advantage of others' misfortune.
4. Misallocation: Buying/selling X causes X or something closely associated with X to be allocated improperly or the wrong way.
5. Corruption: Buying/selling X might corrupt people's character or motives.
6. Rights-violations: Buying/selling X might violate someone's rights.
7. Semiotic: Buying/selling X expresses disrespect for someone or something. (Semiotic objections are the most prevalent in the literature and the most philosophically interesting.)
We might be doing a 2nd edition of the book. It occurred to me while considering this that there is probably an 8th, distinct kind of objection:
8. Associative: Buying/selling X is wrong because doing so associates you with people who are bad or who have bad ideas.
For instance, Guitar Center no longer will sell Fulltone guitar pedals because they regard Fulltone owner Mike Fuller as racist. (Mike Fuller claims instead that he terminated the relationship and that what's really going on is they owe him money.)
Imagine someone refuses to give a talk at a university because the university receives Koch money. Imagine here the worry isn't even that taking the money will help the Koch agenda, but simply that the person doesn't want to be associated with them.
People often think that money is made dirty but association with others.
This seems like a real and prevalent class of objections, especially in light of today's cancel culture. So, we'll probably include a section or two on it.