Historically, I think the point of requiring licensure of a particular profession has usually been to allow that profession to limit entry to its ranks, keeping up prices and allegedly quality. Medieval craft guilds formed spontaneously, and then sought out government sponsorship and enforcement, not the other way around. If landlord licensure followed the same pattern, it would serve to raise rents rather than keep them down. Although as you suggest it would probably also discourage the nastiest slumlord tactics.
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