(6)If any person upon an application made by him has been registered as a dealer under this section, and thereafter it is found that he ought not to have been so registered under the provisions of this section, either because he is not a dealer or because he is not liable to pay tax, lie shall be liable to pay tax on his sales or purchases made during the period commencing on the date on which his registration certificate took effect and ending with its cancellation, notwithstanding that he may not be liable to pay tax under the provisions of this Act.
