Answer: it was voluntary exchange
Explanation: A voluntary exchange is the process where buyers and sellers of products or services freely and without coercion engage in market transactions or exchanges. With voluntary exchange, both parties are better off, and are satisfied when accepting the exchange price.
The principle of voluntary exchange is based on the fact that buyers and sellers are acting in their self-interest, this is evident in the sense that both parties are better off than they were before the exchange. The seller of the traded commodity has a form of payment and the buyer has a product or service.