Internet and agent technologies have facilitated worldwide trade, but we sometimes encounter risky situations in the process of exchange 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 identity in a network is cheap. This makes it almost impossible to exclude malicious agents from trade. One solution is the imposition of an entry fee. However, if the entry fee were too expensive, this would discourage 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. 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 of these mechanisms and provide a calculation method that is able to obtain this value in linear time.