My guess is the bullion dealer would convert to cash, or may try to negotiate with miners to accept Bitcoin as payment. I believe most merchants use a 3rd party (like Bitpay or Coinbase) to facilitate the transaction. Once its completed Bitpay/coinbase converts the purchase to fiat at the point of sale and the customer can withdraw it at anytime.
I have to ask, who is Bitpay and Coinbase? A legitimate bank, Wall Street firm, or just some computer nerds in a basement?