Building trust in e-commerce is especially challenging in complex transactions. Complex transactions, involving a combination of aggregate, optional, and partial transactions, are increasingly relevant in real-world e-commerce scenarios. Existing multi-party contract signing protocols are not suitable for such transactional complexity or rely on trusted third parties (TTPs), resulting in inefficiency and limited applicability. In this paper, we propose a blockchain-based multi-party contract signing protocol specifically designed to ensure trust in complex e-commerce scenarios. Our approach introduces a Pre-agreement sub-protocol which reduces contract signing in complex transactions to contract signing in aggregate transactions, allowing the actual signing process to proceed efficiently and involve only relevant parties. The protocol uses Merkle tree structures to compute a unique identifier for the multi-party contract, reducing computation overhead compared to existing solutions. By invoking smart contracts only in the event of disputes, our solution ensures fairness while minimizing blockchain costs. Formal verification with the Tamarin Prover confirms that the protocol satisfies key security properties, including fairness, confidentiality, and non-repudiation. We also provide an Ethereum-based implementation that further demonstrates its practical viability and cost-effectiveness, providing a trusted coordination mechanism for multi-party contract signing in decentralized e-commerce environments.