Then you're missing the point. You took money from them. You didn't take Newports. You didn't take a goat. You took money, because (a) it's legal tender, so you were required by law to accept it (b) you knew that you could then take it to a different store and use it to buy things because it was legal tender, and they were required to accept it as well.
Dude, if someone brought in gold, silver or diamonds, we would have done business too. Even if it was illegal, even if it wasn't legal tender.
That's the difference between a fiat paper money (which is fiat due to legal tender laws) and real money (that which cannot be counterfeited, and are rare).
Arguments for legal tender are usually pretty nutty. For example, the government could say that paper dipped in bird shit is legal tender. We'd all be dealing with shit dolllars. Or they could say that paper printed with the blood of asian virgins was legal tender. Or they could say that marbles are legal tender. Or cars are legal tender. It's completely arbitrary. In the absence of the law (and how many of us follow the law 100%, answer = none) the good is only worth what someone is willing to trade for it. That's why there is never hyperinflationary or hyperdeflationary gold. But there is hyperinflationary and hyperdeflationary paper.
And as far as the world being the "sufficiently large system" that proves otherwise, you're forgetting that all of the currencies are also legal tender, so your point is lost.
No, my example is valid. Euros are not legal tender in America. But Europeans can exchange them and visit America, and likewise. They do trade with America, and likewise. Same with any other currency.
Credit cards actually make this even easier, as you can convert and pay in the currency of choice on demand at the latest rates. Believe me, if Singapore made gold their legal tender, then you could be able to go to Singapore with zero gold, and still participate in their economy with US dollars.
Again, show a sufficiently large economic system being used in which a lack of legal tender exists.
Done. A lot of very smart people have been talking about this for quite some time. There is a lot of literature on it. It's not quackery. The world is already trending towards nation neutral, legal tender neutral money. Dropping legal tender laws would speed it up tremendously.