TAX PROVISIONS - HARMONIZATION OF LAWS - TURNOVER TAXES - COMMON SYSTEM OF VALUE-ADDED TAX - TAXABLE AMOUNT - TRADE-IN OF SECOND-HAND GOODS BY WAY OF PART-PAYMENT - NATIONAL RULES PROVIDING THAT THE VALUE OF THE GOODS TRADED IN IS NOT PART OF THE TAXABLE AMOUNT - PERMISSIBILITY - CONDITIONS ( COUNCIL DIRECTIVE NO 77/388 , ARTS 11 A 1 ( A ) AND 32 )