There is growing interest in the roles of the sharing economy and grassroots innovation in the transition to sustainable societies. Grassroots innovation research has tended to assume a sharp distinction between grassroots organisations and businesses within niches of socio-technical innovation. However, the non-profit sector literature identifies a tendency for non-profit organisations to actually become more commercially-oriented over time. Seeking to account for this tendency, we develop a conceptual model of the dynamics of grassroots organisations within socio-technical niches. Using a case study of Freegle, a grassroots organisation within the sharing economy niche, we apply the conceptual model to illustrate the causes, processes and outcomes of grassroots niche organisations becoming more commercially-oriented. We show that a grassroots organisation may be subject to coercive and indirect (isomorphic) pressures to become more commercially-oriented and highlight the ambiguities of this dynamic. Furthermore, we highlight that global niche actors may exert coercive pressures that limit the enactment and propagation of the practices and values of grassroots organisations. We conclude by highlighting the need for further research exploring the desirability and feasibility of protecting grassroots organisations from pressures to become more commercially-oriented.