you&#x27;re saying you pay the spread because you&#x27;re taking prices, not making them.  diamonds have a huge spread at retail, and so you get screwed; gold&#x27;s spread is so low that you can actually make money doing this, although more or less only by chance.  i&#x27;d&#x27;ve expected mercury to be more like gold, but so far i can&#x27;t find much evidence of a current functioning mercury market.