The story Gail tells about currency rings true at first, but I don't buy that the link between energy/time/labor and trading value was ever so strong. Even starting with the bartering chair maker and farmer, if the farmer can get a chair for a basket of eggs, he won't give two baskets of eggs for a similar chair to the other chair maker in town just because chair maker B is slow and takes 12 hours to make a chair. Even if there were no fast chair maker around, he may still decide he's better off sitting on the floor or on a stump than paying two baskets of eggs.

The real relevance of time/effort value is this: farmer doesn't pay 1 basket of eggs (6 hours work) for a chair b/c the carpenter spent 6 hours on the chair, he pays 6 hours of work b/c he couldn't have built a nice chair in 6 of his own hours (and there's nothing else he wants/needs more at the moment that he could get with 1 basket).

Commerce has never required anyone to believe (though some may have believed) that a chair was worth a basket of eggs in carbon footprint or in the eyes of God, only that the chair is worth the eggs to me, the farmer.