As Firm 2 buys credits it reduces abatement and its marginal costs fall, as Firm 1 sells credits it has to engage in more abatement and its marginal costs rise. As long as the marginal costs are different the firms have an incentive to trade. In this case, the firms should trade until Firm 1 is abating 80% of its pollution and Firm 2 is abating 20% at which point marginal costs are equal and there is no further incentive to buy and sell.