I think this is an interesting ethical dilemma for a vegetarian -- but it depends on your reasons for being a vegetarian.
I disagree with the responses saying that you might as well buy it because the animal that provided the leather isn't going to come back. You might as well say the same about the meat you see at the butcher's.
If you refuse to eat meat because you are against murdering animals for consumption or clothes, and refuse to have anything to do with what you regard as a corrupt and immoral practice, then you should not buy this car.
If you refuse to eat meat because you do not want any of your money supporting this (supposedly) corrupt and immoral practice, then the fact that the car is second hand may change things. For, as it's second hand, the damage has, as it were, already been done, and, arguably, your money would not go to support the system in any way. On this view, you may be able to buy the car in good conscience.
I'm not entirely sure whether the fact that the car is second hand is enough though. IF there is more demand for second-hand leather cars, then their resell prices will be higher; SOME car buyers may care about the resell value of their cars; accordingly, SOME car buyers may therefore be more inclined to buy a car with leather seats. So there may be an argument to the effect that, even buying the car second hand, contributes indirectly to the meat produce industry.
The effect is very marginal of course. But then, the effect of a single individual's choice not to eat meat is also very marginal!