I don't think this is a fungibility issue. Silver is fungible, the issue is the medium of exchange aspect in this example. Silver is no longer a medium of exchange, thus leading to illiquidity when trying to exchange it for goods and services, or other forms of money.

It's a liquidity problem and a lack of disobedience problem.

Liquidity is not a problem for goods or commodities under high demand. There is a shortage of physical silver to meet the demand.

In the original post the OP was complaining about the bid/ask spread to offload his physical silver in exchange for fiat. He is unable to easily connect with the people who demand the silver, as is his potential buyer who wants to charge him 30%. Volatility will also play into the calculations of the middle man, as will the expected length of time required to offload the physical silver to the true source of demand.