That would, in theory, be possible. Pollution trading credit systems work by having some verifiable way that the seller has reduced emissions, or in legal regimes, that the seller is emitting below some legally set level for a particular activity. In order to work, it must be verifiable, it must be real, and it must also be economical to perform this verification.

That's bad news for car drivers. Since driving a hybrid car is fairly trivial on the emissions reduction scale. 103 gallons of gas produces 1 ton of CO2 which trades for about $4 in US credit markets. So a hybrid driver getting 45mpg over a gas car driver getting 25mpg going 15,000 miles saves 2.6 tons of CO2, or just over $10 of CO2 credit. At European prices it's closer to $30.

But none of these numbers, sadly, are high enough to pay the costs of administering such a program and doing good enforcement and verification.

For 100 vehicles it is a different story, and I don't see why the exchanges would not be willing to do something here. If there were many thousands of people with 100 vechicles, that is. Even $1000 for 100 vehicles wouldn't begin to pay the legal fees to make something like this work if you were the only person asking, I would guess.

Hybrid cars that get up to twice the mileage are good, but the truth is there are far more effective, larger scale ways to reduce pollution right now.

A more likely situation is for a vendor of low mileage cars, like Toyota, to sell credits for what they do. They do it at a scale which can afford the paperwork and enforcement is very simple since they already have all the accounting in place. So instead your Hybrid costs $50 less rather than each individual owner trying to sell credits.

One of the hard parts about pollution trading is figuring out what's a real reduction, the kind we want to subsidize. On one hand, somebody who takes transit is doing ever more to fight pollution, but unless they switch from a car to transit they didn't reduce the total.

One approach is to allocate each person an equal share somehow, and those who want to drive hummers pay more and those who don't commute at all get credits, but the bureaucracy of that is also large.