This paper proposes and critiques the idea of a post-capitalism sustainable consumption utopia to improve the ecological and human wellbeing of the planet. Such a notion can stimulate new imaginative thinking on a future sustainable world not dominated by neoliberalism. It can also strengthen SDG-12: responsible consumption and production. To do so, it examines the influence of pro-environmental self-identity, market-based barriers, and knowledge barriers on sustainable consumption buying, product lifetime extension, and environmental activism. Survey data was collected via online panels in Sweden (n=504) and the USA (n=1,017). Richly varied and complex findings emerge supporting the merit of this utopian idea. In particular, the importance of pro-environmental self-identity. This study illustrates how the post-capitalism notions of radical incrementalism and people power can initiate change using the civic, political, and environmental activism in sustainable consumption behaviours. Emerging implications for the viability of SDG-12 are also considered. This work offers rich opportunities for further research.