If two firms have different abatement costs, in a system of marketable pollution permits:
◦ the firm with lower abatement costs will purchase permits from the firm with higher abatement costs.
◦ the firm with lower abatement costs will sell permits to the firm with higher abatement costs.
◦ voluntary exchange will occur, but it is impossible to determine which firm will be the seller and which will be the purchaser.
◦ no voluntary exchange that makes both firms better off is possible.