Folks, "snake-oil salesmen" isn't a term of endearment. Snake-oil may not be an herb, but that's pretty much what this dude is selling. Oh, you're not buying? Well, allow him to convince you that you should be.
No wonder this dude gets some serious side-on on this ship.
That said, the herb-doctor is super careful about his promises; we're assuming this guy is a con-man, sure, but he sure knows how to pull off a good con. Basically, everything he promises sounds pretty much legal and aboveboard and maybe even healthy, to boot. Plus, he deliberately distances himself from mineral doctors (another version of snake-oil salesmen), who he says are not the real deal:
"Still, while for the world's good I refuse to further the cause of these mineral doctors, I would fain regard them, not as willful wrong-doers, but good Samaritans erring. And is this—I put it to you, sir—is this the view of an arrogant rival and pretender?" (16, 4)
This might be a pot-calling-the-kettle-black situation, but it also shows that the herb-doctor is thinking two steps ahead. He knows that medicine peddlers are viewed with suspicion, so he raises the issue and tries to argue that he's not like those other guys. Just have a little confidence in him and give his herbals a chance…