GST on Second Hand Mobile

My Client buys 2nd hand mobile (For Rs 12000) from unregistered person and then resell it (For Rs 15000) to normal customers.

Under Margin Scheme We have to pay GST on Difference (15000-12000=3000)

I have following ques
ry;Please help
1.) How to report this in monthly GST Return ?
2.) In GST 3B Return Where to show 15000 amount whereas in taxable I have to show difference i.e 3000 ?
3.) What would be his turnover 15000 or 3000 ?
4.) How to issue such Invoice to customer ?

Please advice or correct me if I am wrong in above case.
Regards