As electric utility systems around the world continue to move towards open, competitive markets, the need for new modelling techniques will become more obvious. To study electricity markets behaviour and evolution we propose a multi-agent simulator where agents represent several entities that can be found in electricity markets, such as generators, consumers, market operators and network operators, but also entities that are emerging with the advent of liberalization, such as traders. The simulator probes the possible effects of market rules and conditions by simulating the strategic behaviour of participants. In this paper a special attention is devoted to the strategic decision processes of Seller, Buyer and Trader agents, in order to gain advantage facing the new emerging competitive market.