We address long-term coalitions that are formed of both customer and vendor agents after evaluating their relationships with other agents in the system. We present a coalition formation mechanism designed at agent level and analyzed at both system and agent levels. Evaluation has been conducted to compare agent strategies (individual vs. social) and to analyze the system behavior under different circumstances. Our results show that the coalition formation mechanism is beneficial for both the system (it reaches an equilibrium state) and for the agents (their gains increase exponentially in time).