Recognizing the responsibility businesses has in sustainable development, a rising number of entrepreneurs are attempting to innovate business models together with digital technology to address environmental and societal needs. Circular business models is an example of how businesses can become more sustainable. However, there is a growing phenomenon of entrepreneurs suggesting digital platforms as a supportive element in an enterprise to enable circular features. This research aims to investigate the role of a single entrepreneur as a transition intermediary to sustainable development by using digital platforms. Additionally, this research aims to explore the success factors and challenges this entrepreneur can uncover. Contrary to most studies, this thesis adopts the entrepreneur's perspective as being a key actor in sustainable development but also as an innovative force in a socio-technical system. A case study was conducted on the Swedish online secondhand store Sellpy. The qualitative data were collected using semi-structured interviews. Given the thesis' exploratory research design, the finding can be used in further research as artifacts for more conclusive and generalizing research. The entrepreneur, as a transition intermediary, can establish valuable partnerships and networks to accelerate circulation and sustainability awareness. Additionally, the entrepreneur can influence others by spreading knowledge to engage other entrepreneurs to innovate businesses towards sustainability. The main success factors found in this case study is the approach to develop with the user community, digital ownership, skilled and diverse workforce, and partnerships with similar businesses. The challenges discovered was to maintain and find new users to enter the circular system and the external skepticism towards sustainability-oriented businesses.