"If you don't believe me, consider why the US dollar has value at your local store and the Monopoly dollar doesn't."

Well, the primary reason is that US dollars are declared by government fiat (hence the term) to be "legal tender for all debts public and private". You can't legally refuse to take US dollars (in the United States, obviously ... other countries give their own currencies a similar status). That seems only tangentially related to whether the dollars come from taxes or printing presses.