# A Novel Decentralized E-Commerce Transaction System Based on Blockchain
## 1 Introduction  
With the increasing popularity of Internet technology, commodity exchange has undergone profound reforms and innovations, gradually transforming from traditional offline exchange to online real-time transaction [1]. The sales volume of retail e-commerce achieved totally USD 4.89 trillion in 2021 and is expected to grow to USD 6.39 trillion in 2024 [2]. E-commerce has fundamentally changed people's way of life, shopping, office, medical, education, entertainment and other necessities of people's lives can be perfectly solved through the e-commerce system. The third-party network payment platform driven by the e-commerce industry has fundamentally overturned the original capital flow and brought greater convenience to people. However, current e-commerce platforms face some problems. For example, (1) the third-party organization (logistics party) is supervised by the platform only, probably leading to false logistics information and advanced signature [3]; (2) commodity data and buyer information are stored in enterprise servers centrally, which are vulnerable to be maliciously attacked, resulting in data leakage or theft; (3) Lack of incentive mechanism, buyers have an inactive enthusiasm for comments, and there are numerous false praise and comments, influencing buyers' judgment towards sellers. Blockchain is a distributed ledger consisting of encryption algorithm, consensus mechanism and smart contract, featured by authenticity, unforgeability and traceability [4, 5], whose core value lies in establishing the open and transparent rules of the algorithms,constructing a trust network [6], and ensuring the security and privacy of transactions [7]. The combination of blockchain and e-commerce transaction systems can enhance the trust between users and platforms, protect the privacy of users and ensure that their evaluation cannot be modified, improving the reliability, security and credibility of the system [8; 9]. In recent years, decentralized reputation systems (DRSs) have been proposed. However, researchers focus on decentralized reputation management for simple transactions rather than online shopping, and do not sufficiently consider transaction factors such as transaction time, transaction amount, and user's previous reputation score to prevent common attacks when performing reputation assessment. Therefore, this paper proposes an e-commerce transaction system based on blockchain and IPFS as well as a reputation value calculation model based on Multi-Criteria Decision Making (MCDM). Specifically, the seller stores the detailed information (name, price, etc.) of commodity in the IPFS system and obtains the return address in the chain. The buyer concludes the transaction through the smart contract [10]. The logistics party updates the logistics information on the blockchain, which is then signed by the seller for evaluation. The platform can calculate and update the reputation value according to the transaction records. The main contributions of this paper are as follows. 1. This paper proposes a new e-commerce transaction system based on blockchain. The buyer and seller temporarily store the deposit in a smart contract, and the logistics service provider updates the logistics information on blockchain [11]. When the buyer receives the goods, the deposit will be automatically transferred to the seller. 2. Unlike most decentralized e-commerce systems, this paper proposes to store the item details in IPFS and then store the item address in the blockchain. To reduce IPFS storage space, commodity records are periodically cleaned up. 3. By investigating the common attacks on reputation evaluation, this paper proposes a reputation evaluation scheme based on simple weighting (SAW) and MCDM. By combining different factors (transaction completion time, reputation value of trading partners) to evaluate the reputation value of the participants, it can effectively defend against various attacks, such as collusion attacks and unfair evaluation attacks against reputation value. 4.