This might be completely rational if there was an excess of perfectly good used cars out there, but there isn't. Cars hold their value incredibly well compared to almost any other consumer product. We're not talking about $30 DVD players. Any used car worth a damn will get sold and bought up. There is almost always someone willing to take up the slack and keep a car running for decades. Therefore, by buying a new car, you're simply decreasing the demand for used cars slightly and lowering their price, which will just mean that someone will just get a better deal.

The only way to effectively reduce carbon emissions on the production side is to drive less person-miles. Buying used or new doesn't change much.