Why would you only be able to sell gold and silver to the banks, for the price they dictate?
Just that is the big difference of gold and silver to fake currenies: gold and silver is made of gold and silver, not of paper or cheap metal. And that doesn't change, no matter what laws they issue.
So if someone want to get gold or silver - for example a jewellery maker - where would he he rather buy it from? From a bank for a extortionate price combined with a report to the IRS, or tax free for a generally accepted price from a unknown seller? You see where I'm going ? Gold and silver will always keep its value, legal or not. The more opressive laws get, the less people care about obeying them.