He was knowingly distributing it to people who were going to use it to self-delete. Ultra libertarians may defend this, but most people probably think that selling a means of CTB to a depressed person is a rather reprehensible action.
From a legal point of view, things are more interesting. SN is not illegal, and he did ask for proof of age before sending it (though I've heard that this was not strictly adhered to). A good lawyer could possibly get him off the hook based on that.
In my opinion, this should be taken as just one more reason as to why legal euthanasia should be a thing. People are desperate enough to CTB to the point that they are buying chemicals online from shady folks like this man. Abortion was finally legalised after decades of women's seeking dangerous illegal abortions because it was accepted that it is something that, whilst horrible, is just going to happen. Suicide is the same.