When two people each have items the other wants, both people can determine the values of the items and provide the amount that results in an optimal allocation of resources. Therefore, if an individual has 20 pounds of rice that he values at $10, he can exchange it with another individual who needs rice and who has something that the individual wants that's valued at $10. A person can also exchange an item for something that the individual does not need because there is a ready market to dispose of that item.
For instance, let's say your pipes burst at your home and you find yourself in need of a plumber. It's an emergency situation and if you don't hire a plumber fast, you'll be displaced and will have to move your family out of the house and into a temporary living situation. That certainly would be a bad thing. But should you barter your dog grooming services right now? Do you really want to trust your plumbing to the only person in town who'll do the job if you shampoo their poodle?