Patty O'Rourke hired an attorney to represent her in a court case involving an auto accident. The attorney charged O'Rourke a fee for his services. Terry Thomas needed a haircut--the local stylist charged him $12 for her services. Aaron Mathison mowed his neighbor's lawn; in exchange, the neighbor roto-tilled Mathison's garden. The attorney fees paid by O'Rourke, the $12 charged by the hair stylist, and exchange of lawn mowing for garden tilling are examples of: