Sir,
According to rule 32 (5) of CGST Rules, 2017 "where a taxable supply is provided by a person dealing in buying and selling of second hand goods i.e., used goods as such or after such minor processing which does not change the nature of the goods and where no input tax credit has been availed on the purchase of such goods, the value of supply shall be the difference between the selling price and the purchase price and where the value of such supply is negative, it shall be ignored."
Now the answers for your query are as under: -
1. Second hand dealer can export second hand goods provided it i is acceptable to the recipient of the goods.
2. If the second goods are exported on payment of IGST then the exporter can claim refund of tax paid.
3. the value for export is the transaction value as indicated in the export invoice.
However, one problem in margin scheme is that the seller has to declare his purchase price in the export invoice and no seller would like to disclose his margin to the recipient.