In rationing system, you need purchases operate on a two-key system. To buy something first you need the money, then you need the coupon. So even if you had the money you couldn’t buy without the coupon. Now suppose someone had coupons he didn’t want, then those coupons were available for trade in some way. You could buy the coupon off somebody and then buy the product. So if a pound of butter cost X dollars, the off-coupon cost of a pound of butter was X dollars + the coupon price. Essentially you get penalized for every consumption above the rationed level.

But in the case of wartime rationing, there was an physical ceiling on the amount of goods available, as determined by how much shipping could get through the U-Boat attacks. In the case of carbon credits, there’s really no market limit. The air ticket, for example, is actually available. But it is off limits if you can’t buy it without a ration coupon. I might be wrong, but I think it alters the money supply because not all currency is equally spendable. Imagine, as a thought experiment, a situation in which the sum of ration coupons issued were a fraction of GNP. Then despite any trades you could effect, in the net there would come a point where you couldn’t buy anything with your money. I suppose you could save or invest it, maybe solar panels or some thing like it.