The rise of online flea markets in Japan has expanded access to secondhand products. Despite this expansion and the increasing recognition of the need for sustainable consumption, there remains a notable constraint on consumer acceptance of pre-owned items. Thus, this study emphasizes the utility of secondhand product usage in a context where consumer acceptance of pre-owned items is limited despite the recognized sustainability necessity. Additionally, it aims to investigate secondhand clothing-utilization patterns among users, estimate the environmental potential reduction, and assess their contribution to sustainable consumption. Conducted on users of the Japanese online flea market platform, Mercari (sellers: n = 13,325, buyers: n = 9985), the study quantified the potential reduction in greenhouse gas (GHG) emissions by reusing secondhand clothing. Utilizing a survey, the key findings revealed distinct reuse characteristics for different items, confirming extended product lifespan even for those in poor condition. Moreover, secondhand products were used more frequently than new ones, and buyers exhibited less concern about potential damage. The displacement rate of secondhand for new clothing did not significantly decrease with worsening product condition. The study estimated a potential 9.6 thousand tons of carbon dioxide equivalents (ton-CO2eq) reduction under the assumption of 10 million secondhand clothing items replacing new ones. This study proposes the feasibility of utilizing secondhand items and their potential contribution to environmental conservation in consumption. Further, it establishes the theoretical foundation necessary to articulate the environmental significance of secondhand products. Simultaneously, it provides valuable insights into the broader implications of sustainable consumption practices.