Typically, online transactions require a significant level of human intervention. Customers undertake the search for products, services, potential vendors and business partners. They evaluate alternatives, decide what goods to buy and when, from which vendor, and how much they are willing to pay for. They engage in potential negotiations, carry out the transactions and so on. As those markets become mature, new products and services, as well as new practices, are beginning to appear. There is already ample evidence that agent-based technology will be crucial for these developments, reflecting that both buyers and sellers being vigorously seek techniques to improve their performance. This work surveys several of these agents that help electronic commerce systems by describing their roles in the context of a Consumer Buying Behavior (CBB) model.