With the growing number of marketplaces and trading partners in the e-commerce environment, software tools designed to act on behalf of human traders are increasingly used to automate trading activities. This paper describes a model for constructing trading engines which are capable of concurrently participating in multiple interrelated negotiations with heterogeneous protocols. These tree-structured engines are configured by means of a single generic synchronization construct which enables the incremental composition of complex trading schemes, including a number of well known strategies from the financial trading domain. The construct is augmented by a priority-based scheduling algorithm which selects a set of nodes for negotiation based on their estimated profit, the time remaining and the desired degree of concurrency. The model also provides iterative negotiation, which is essential in any complex trading environment.