Final answer:
Media buying in the third-party exchange marketplace model represents a horizontal market, which is characterized by services that target a broad range of industries rather than a specific sector.
Step-by-step explanation:
In the third-party exchange marketplace model, media buying is an example of a horizontal market. A horizontal market is one where products and services are offered to a wide variety of industries and consumers, rather than focusing on a single industry or niche. In contrast, a vertical market would be one that caters to the needs of a specific industry or demographic. Media buying across various industries exemplifies this broad appeal and applicability, making it an accurate illustration of a horizontal market in the context of a marketplace model.