“if an assessee establishes that a particular turnover represents second sales of goods taxable at the point of sale at the first sale and the first sale was by a dealer liable to pay tax, then the Revenue cannot deny the exemption on the second sales merely on the ground that the registration certificate of the first seller was cancelled during the assessment year in question or immediately before the purchase by the assessee, namely, the second seller. We must, however, add that the assessee (second seller) must establish that the first sale was a taxable sale and whether the tax had been paid by the first seller or not is not the concern of the assessee (second seller). We would like to add that the Revenue cannot take advantage of the fact of cancellation of the registration certificate of the first seller either on his application or otherwise, without further investigation as to whether the bills issued by such dealers were real or bogus bill. In the event of the Revenue establishing that the bills produced by the assessee (second seller) are bogus and were not really issued by the first settler, then the assessee cannot claim exemption on the alleged second sales. In that situation, the assessee's sale will be the first sale.”