Global e-commerce has dramatically transformed retail over the past few years, but it has brought along a number of environmental and social challenges, not least in the area of product returns and waste. In this study, the authors explore the possibility of combining RL and CE practices into the ecosystem of e-commerce startups, where managing resources is difficult, while being operationally agile is necessary. The research uses the systematic literature review (SLR) methodology to explore and analyze the latest academic research and industry practices over the period of 2019-2025. According to key findings, the return rates of e-commerce startups are relatively high; as high as 30% in some cases, and that can cause a significant loss of profits if the return process is not handled in a sustainable way. The research shows that Industry 4.0 technologies such as Artificial Intelligence (AI), Blockchain and the Internet of Things (IoT) play a key role in optimizing reverse logistics networks and enable product refurbishment and recycling. The practical implications for start-up founders are related to the implementation of circular business models, including second-hand resale and product-as-a-service, as ways of sustaining the growth and build customer loyalty. This study is original in its multi-dimensional approach toward sustainability focusing on the startup context and pointing to strategic pathways for making the transition from linear to closed-loop supply chains.