Internet and agent technologies have facilitated world-wide trade, but we sometimes encounter risky situation in exchange processes involving goods and money, i. e., fraud. This problem is becoming more serious with the growing popularity of person-to-person trade. One of the reasons for such fraud is that obtaining a new identifier in netwoeks is cheap. This makes it difficult to exclude malicious agents from trade. One solution is the imposition of an entry fee. However, if an entry fee is expensive, it discourages newcomers from starting deals. To resolve the conflict between safety and convenience, we have developed two kinds of exchange mechanisms that guarantee against defection from a contract. One reduces the entry fee by integrating multiple deals and controlling goods and money flows. We also show an extension of this mechanism that can reduce the entry fee more while taking a longer time to complete the exchange process. The other reduces the entry fee by incorporating a third party agent into the exchange process. We examine the lower bound of the entry fee for each mechanism and provide a calculation method that is able to obtain this value in linear time.