A trade certificate shall be used only by the person to whom it is issued and such person shall not allow or offer or cause the certificate or the number assigned in connection there with to be used by any other person:
Provided that the provision of this rule shall not apply where the person to whom the certificate is granted, or a person bona fide in his employment and acting under his authority, or any other person bona fide acting on behalf of the holder of a trade certificate is present in the vehicle, or if such vehicle is designed for use by only one person and is being used by a prospective purchaser of that vehicle for the purpose of reasonable test or trial.