It's not the fact that he was selling gold or silver coins that's problematic. What's problematic is that the coins do bear some similarity to actual US coins. If he would have stamped Snoopy or Bugs Bunny on the coins, he wouldn't have been convicted.
The government alleged and argued (amongst other things) that the coins looked too similar to actual currency. The jurors agreed.
Jurors will agree to all kinds of stupid things.
What’s that company that sells all sorts of silver and gold coins that really look like US coins? How do they get away with it?