The car you’re buying has already been made, it’s carbon been produced, and now you’re generating less emissions with the newer more efficient vehicle

Actually, no. If millions of cars are sold it doesn't mean that all of them immediately popped in existence, materials brought, wages paid and emissions produced. They do them in batches and scale production based on demand. One person not buying a car might not make a dent, but a thousand will. So, while the carbon emissions of that car you see at a dealer's has already produced, by buying it you're giving manufacturer the funds to produce the next one, effectively the same as if you've enabled the carbon emissions of that car in the first place.