This vocabulary subset looks 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, one from each direction. 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.
This differs from (for example) the Good Relations Conceptual Model, which we like and want to intertrade with. But Good Relations assumes Compensation in the form of money; the compensation itself is not a separate promise; and the actual flow events are not part of the model. (That's not a criticism. Good Relations has different goals, and a more minimal model makes sense for them.)
In ValueFlows, we want to track not only the offers and promises, but also the actual flows of resources in networks, in all directions. And we want to support exchanges that don't involve money as well as those that do.