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.
The transaction itself is not illegal. I concede that point. But not paying the taxes on the transaction is illegal.