## 1 Introduction  
In contemporary society, the pursuit of sustainable consumption and the imperative of environmental protection stand as pivotal concerns. The excessive consumption of energy coupled with the rapid, iterative development of products poses formidable challenges to the goal of achieving sustainable development [1, 2]. In particular, consumer culture is driven by the quest for immediate gratification, which engenders resource wastage and environmental deterioration in addition to enduring adverse consequences [3, 4]. For example, rapidly changing fashions and the fleeting pursuit of particular design styles have led to the trend of disposable product use. This trend not only results in the continuous growth of solid waste but also has numerous negative impacts on the environment and climate change [5]. According to an analysis by _Business Insider_, fashion production accounts for 10% of global carbon emissions, and 85% of textiles are thrown away each year [6]. Over the past 15 years, the number of times a piece of clothing is worn has decreased by more than one-third, to an average of seven times, leading to the generation of 92 million tons of textile waste globally each year [7]. In this social context, second-hand trading apps are playing a crucial role in providing consumers with new consumption experiences while simultaneously fostering a culture of sustainable consumption [8]. From the perspective of sustainability, second-hand goods trading not only breathes new life into products by allowing unneeded or idle items to be passed on to those who can use them, effectively extending product lifecycles; it also significantly reduces resource consumption and waste output [9]. According to a report by Schibsted, the buying and selling of second-hand items through their platform resulted in approximately 290,992 tons of steel saved in 2021, equivalent to the amount used in 11.5 million bicycles, and a further 8.2 billion plastic bags were saved simply through use of the second-hand trading app [10]. With the ubiquity of smartphones, the methods of second-hand trading have become more diversified, and second-hand trading apps have emerged as important platforms for users to conveniently buy and sell used items [11]. However, according to one report, more than 56% of users uninstall apps within the first seven days of installation [12]. Therefore, optimizing the app's usability becomes especially crucial to attract and retain more users for second-hand trading apps [13]. However, assessing and improving the sustainable usability of second-hand trading apps remains a challenge, and the development of usability evaluation metrics wherein both user convenience and environmental impact are considered is an important research topic [14; 15].  
In order to achieve the above objectives, this study addresses the following research questions:  
RQ1: Which indicators are closely related to the usability evaluation of a sustainable second-hand trading app?  
RQ2: What is the importance ranking of indicators at different levels in the usability evaluation of a sustainable second-hand trading app?  
RQ3: What is the overall importance ranking of the indicators in the usability evaluation of a sustainable second-hand trading app?  
In summary, the purpose of this study is to rank the importance of usability indicators for second-hand trading apps using the Delphi method, Exploratory Factor Analysis (EFA), and Analytic Hierarchy Process (AHP) to identify the key indicators that enhance usability. The findings will not only aid in developing applications that better meet user needs, improve user experience and satisfaction, and reduce the uninstall rate but also have profound implications for encouraging users to adopt eco-friendly living habits, mitigating environmental pollution, and reducing resource wastage.