Keyword being LEGAL tender.
You are misunderstanding the term "legal tender".
A medium of payment allowed by law or recognized by a legal system to be valid for meeting a financial obligation.
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http://en.wikipedia.org/wiki/Legal_tenderSo that doesn't mean using anything else to pay is illegal, it simply means I cannot force you to pay in anything other than the medium defined by law as legal tender. If you and I want to trade using seashells (which are anonymous, and can be traded anymously), that is not using legal tender but it most certainly is not prohibited by law.