Ectomycorrhizal systems are marked by an exchange of carbon from the plant for soil-derived nutrients (chiefly N) from the fungi. This exchange can be viewed as resource trading within a biological market, and analyzed using tools of microeconomics. Here I present a mathematical model that addresses the C allocation decisions of a single plant trading C for N with a community of associated ectomycorrhizal fungi. Specifically, I focus on the role of C for N trading in determining fungal coexistence, species composition, and shits of fungal species composition with changes in light availability. In an added layer of complexity, the C for N trading operates in the context of fungal population dynamics, and competitive (allelopathic) fungal interactions. I discuss how these forces may distort the market dynamics, and alter patterns of coexistence and relative abundance in mycorrhizal fungal communities.