The growing controversy around the adverse impacts of sharing economy platforms, including Airbnb and Uber, has led to calls for more democratic models of platform governance. Advocates of democratic governance models claim that these could help to create a more sustainable sharing economy by ensuring that platforms promote social and environmental values alongside the instrumental values of the capitalist economy. Exploring this claim we bring together theories of platform governance, democratic organisational governance and the enactment of values in socio-technical systems, offering a conceptual framework for analysing emerging democratic models of platform governance. Applying this framework, we present a mixed methods case study focused on Freegle, a platform that enables people to gift unwanted consumer goods locally and one of very few platforms to have sustained a democratic governance model at scale. In particular, we explore the extent to which the social, environmental and instrumental values of platform users and owners have been accommodated in the governance of Freegle. Our findings are broadly supportive of the claims made by advocates of democratic platform governance, although in Freegle's case social and environmental values are rather better accommodated than instrumental values. While we are cautiously optimistic about the potential from a sustainability perspective of democratic platform governance models, we emphasise that major challenges are likely to be faced by those implementing these; and that such models should be complemented by government regulation of the sharing economy.