Final answer:
Joy is acting as a fiduciary for David by selling his rare coin collection at the local coin show and securing the best deal due to her expertise in rare coins.
Step-by-step explanation:
In the scenario described, Joy is acting on behalf of David as a fiduciary. A fiduciary is a person who holds a legal or ethical relationship of trust with one or more other parties. In this case, Joy is expected to act in David's best interest when selling his rare coin collection at the local coin show. Her expertise in rare coins implies that she has the knowledge and skills to secure the best deal possible, hence fulfilling her fiduciary duties.