Section 201. Imposition.--A tax is hereby imposed upon each separate sale at retail within this Commonwealth, which tax shall be collected by the seller from the purchaser and shall be paid over to the Commonwealth as herein provided. In the case of a sale at retail of motor vehicles, trailers or semi-trailers by one not a seller, the tax shall be paid by the ultimate consumer to the department upon application to the department for the issuance of a certificate of title. The department shall not issue a certificate of title until the tax has been paid or evidence satisfactory to the department has been given to show that no tax is due.