We all have to understand how this rate of 18% is arrived at. Basically, in the present system, there are two principal taxes - Excise Duty and the Sales Tax. The Excise is charged by the Central Govt. while Sales Tax (or VAT) is charged by the State Govt. On most of the items, the Excise is 12.5% and the VAT is 5%. So the total comes to 17.5%, which is proposed as 18% under GST. The Excise Duty is always charged on the MANUFACTURING of an item, while the VAT or Sales Tax is charged on the SALE of an item. Now the point in our (NUMISMATISTS or COIN COLLECTORS) favour is that the ancient coins are not MANUFACTURED now, and hence are not liable for the payment of Excise Duty part. Yes, for the coins being sold by the Mints, this theory will not apply, and there the rate of 18% GST can be justified.