Fairs of family farming on agroecological bases are direct sales channels of great importance for local supply. Thus, it is essential to devise ways to evaluate and monitor their evolution. In the present case study, a methodology was applied to analyze commercialization at the Family Agriculture Fair (FAF) held at the Seropédica Campus of the Federal Rural University of Rio de Janeiro (UFRRJ). The objective of this study was to analyze gross sales and the seasonality of the supply and demand of products in the FAF, relate these factors to agroecological production in the Baixada Fluminense, and both demonstrate and understand the relationships among the supply, prices and sales of products. A literature review was conducted on the short circuit and commercialization forms of family farming, and data from 1,664 marketing reports, self-completed by market traders from 2017 to 2019, were analyzed. The results allow us to conclude that (i) the stallholders farmers at FAF are not mere price takers; (ii) gross sales are influenced by the seasonality of demand and supply; (iii) the effects of seasonality have an impact on supply and sales but are reduced with low price volatility; and (iv) the offers are diversified, and the items have close substitutes within the same product category.