GST on Sale of Old Gold Jewellery(2nd Hand)

My client (GST Registered) is dealing in purchasing & selling old gold jewellery (2nd hand jewellery).

They purchase Old Gold Jewellery from unregistered person and then sells it to customer after minor repairs & polish (No change in the form and nature of goods).

1.) How much GST he should pay ?
(Full Sale Value or Difference b/w purchase price & selling price)

2.) How to invoice such sale? (Plz. give an example)

3.) How much would be his turnover in GST ? (Sale Value or Difference)


Kindly help.
I shall be very thankful.