I will reply by telling you two things:
1.- In China, no two cities may do things the same way or apply laws and regulations the same way. Hell, in the same city, it will also depend on person involved, and day of the week too.
2.- In Nanning at least, license plates are really tied up to your bike. In fact, when you get the plate, the police takes a photo of it placed on the bike for the record. If your bike is stolen, you must get the manufacturer of your bike to make one with the same color, etc, even with the stolen one serial number for you to get the replacement license plate. Again ,at least here, it would be very difficult to fing a seller of a license plate. And if you find one, and use the plate in your bike, if the police has a need to check on you and your bike for any reason, the truth will come out very quickly.

Don't buy a motor vehicle without having the papers sorted. If there is no license for it from the sellers side, walk away. Sure with the vigilance of China's finest, the chances of getting caught with a stolen bike or stolen plates is minimal. But still, go to the motor registration office and get plates there. If the bikes serial number doesn't check out, well, then hope the guy who sold you the bike is still to be found. 

Brother, you can buy anything in China. Risk-free?  Never.  But as Scandinavian pointed out you probably have a 1 in 10 chance of getting caught.  

Tags are sold on the gray market every day.  Your call may depend on how long you plan to stay in one city and your driving habits.  My bike is a mongrel - built from various bikes so by necessity I had to buy a tag. I always drive where I don't have to stop at red lights - up front where the traffic cops tend to station themselves.  Gong on four years now without a problem.  When you go on highways however, your chances of detection jump up to about 20% as they sometimes do random checks on exit ramps.