There is nothing about a "commonwealth" state that changes anything with respect to the law. About all you can say about a "commonwealth" state is that it's a "commonwealth" state.
You are missing the fact that you CANNOT allow a buyer to use your registration. The registration is effectively cancelled as soon as the vehicle has been sold. You can't hold a registration on what you no longer own, therefore it is invalid.
And, as I've stated previously, any reasonable person knows that they are buying the car, not the registration. It is unreasonable to expect that you can buy a car and continue to use it with the previous owner's registration.
Not even getting into the insurance aspect of it, which is another problem. Most states require a car that's registered to also have a valid insurance policy.
Is the seller supposed to keep the insurance in force? Is the irresponsible buyer who hasn't bothered to title or register the car in their name going to get insurance on it?
And, let's say the buyer gets pulled over, while the vehicle is still displaying the plates registered to the previous owner. Do you really think claiming ignorance is going to get the buyer out of the heap of trouble he's going to be in?