Trust evaluation is critical to peer-to-peer (P2P) e-commerce environments. Traditionally the evaluation process is based on other peers' recommendations neglecting transaction amounts. This may lead to the bias in transaction trust evaluation and risk the new transaction. The weakness may be exploited by dishonest sellers to obtain good transaction reputation by selling cheap goods and then cheat buyers by selling expensive goods. In this paper we present a novel model for transaction trust evaluation, which differentiates transaction amounts when computing trust values. The trust evaluation is dependent on transaction history, the amounts of old transactions, and the amount of the new transaction. Therefore, the trust value can be taken as the risk indication of the forthcoming transaction and is valuable for the decision-making of buyers.