Your not actually earning until you cash out your Steem for fiat currency. So when you cash out just keep a full record of the amounts so you can calculate the correct percentage if they ask you.

The best way to sell silver for Steem is probably via Raffle to get the best deal like @pit-bullion, or just make a post showing the silver and what you want for it including postage costs.

This post would have been more eye catching if you included some pictures of the shiny stuff here to wet our appetites :-)