Is it legal for private citizens to rent out vacant housing units in China? If so, that doesn't surprise me very much. Every landlord has more housing units than they can use.
Yes it's legal but the people I've talked to don't do it. There's a stigma of renting over buying so the rent/home value ratio is way off compared to the US. Renting out the homes wouldn't really net much profit overall and they don't want to deal with being a landlord anyway, so they let the apartments sit empty.