Trading them yes, but that only excludes trading them, not having them, accepting them as gifts, organizing your society around feudalism and serfdom, etc. (At one point Buddha says monasteries must accept gifts of slaves, other times that individual monks who must not accept them. He is sometimes maybe inconsistent.)
Morality and ethics aren't the same thing. It has the first (things you should and shouldn't do) but not the second (a modern system of reasoning explaining why not to do these things).
> It has little or no “ethics”: broad principles which explain why particular actions and traits are good or bad.
This isn't true. Acting in certain ways is not to be done because it causes misery to those you inflict it upon. The Buddha explains why you should act ethically: because it causes you distress and others distress.
This article is of poor quality. It appears most critiques of Buddhism wish to misunderstand it right from the beginning. Quoting other academic scholars who also misunderstand Buddhism, as your article does, is not making sound arguments.
Here's just one entire sutta on good and bad ethical action and why one should act in this way:
> ‘If someone were to distort my meaning by lying, I wouldn’t like it. But if I were to distort someone else’s meaning by lying, they wouldn’t like it either. The thing that is disliked by me is also disliked by someone else. Since I dislike this thing, how can I inflict it on others?’ Reflecting in this way, they give up lying themselves. And they encourage others to give up lying, praising the giving up of lying. So their verbal behavior is purified in three points.
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