Here we look at exchanges of resources from an independent or neutral viewpoint (not the viewpoint of one of the Agents in the exchange). For example, from one Agent's viewpoint, the exchange may be a Purchase, from the other Agent's viewpoint, it might be a Sale. From the neutral viewpoint, it is an exchange of resources, with usually at least two flows of resources, from different directions. So for example, the seller might give some goods to the buyer, and the buyer might give some money to the seller. Or in a barter exchange, one agent might give the other some books, and the other agent might compensate with some cookies.