basically barter system is the ultimate supply and demand. Say i only have chickens, and eggs. But i need wicker baskets for eggs, but the wicker woman only wants cows, then i'd have to find someone who needs eggs or chickens and has cows, but the only one who wants chicken only sells pigs...... And so on.
With money, I sell my chickens to the pig person, and buy wicker from the wicker woman, the pig person now has chickens and sells his pigs for money, while the wicker woman has money for a cow.
The little green pieces of paper aren't the problem. It's the system of nonexisting money we started building since thew 50's