What mearrin69 said.... regardless of if you use paper(dollars, trade notes, etc), metal(gold, silver, iron, etc), salt or whatever, you really should consider some form of currency as a standard index for trading. Even if two people are trading goods vs goods, it's still nice to be able to put a "price" index upon the things being traded have equal value by weight/volume. As suggested above, 20 lbs of rice should not be worth the same as 20 lbs of wood or 20 lbs of silver. While the rice takes a long time to grow, the time invested vs the harvest is very small (assuming an abundance of "seeds" and growing habitat as there would be here on earth.) Also notice that while rice is fairly inexpensive anywhere(thinking in our world) in most cases, it should be worth far more the further you get from the origination source. I would expect that you could make very little money selling rice in a city where there are hundreds of acres of it around and far more in a more arid region where there is not enough moisture for growing it.