It needn't be illegal to be disallowed. Apple provides this service at will, and can set the terms in which you use it. There are, of course, legal boundaries within which every company must operate, but I'm not familiar with Chinese consumer protection law, so I have no idea if this is inside or outside the law.

Also, it's not that brilliant, IMO. It is not unusual for secondary markets to spring up when the commodity is fungible and there is a supply constraint. See also: scalping sporting event and concert tickets; a practice that is also seen as a shiesty thing to do.