banks and nbfc are only custodian of old gold jewellery. .so the are not allowed to trade in it..they can auction on as is where is basis ...only..and make bidders bill as old gold jewellery. no gst must be charged....as custodians are not allowed to trade in sale or buying of old gold jewellery by RBI rules n regulations .if they charge any gst they will have to payback the gst to their clients with penalty and interest by law.