The actual problem is that the entire concept of trying to separate currency from commodities is defective. Forget about used iPhones. You can walk into any pawn shop and exchange a bar of silver for goods or cash. Does that mean silver is a currency? What about copper? Or gasoline? Try to buy something on Craigslist and offer to pay with a gas card. More likely to succeed than offering bitcoin.

Stocks, which most people don't understand and can't be sold without contacting a broker are considered liquid.

Jewelry & stamp collections are not considered liquid assets, yet everyone understands what these are and where to purchase them.