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.

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Firm 1 % Abatement	MCA	Firm 2 % Abatement	MCA
10	10	10	70
20	20	20	80
30	30	30	90
40	40	40	100
50	50	50	110
60	60	60	120
70	70	70	130
80	80	80	140
90	90	90	150
100	100	100	160