It's a people person job. The point of selling cars is making the relationship with the buyer. It's the salesman that really moves the buyer to make the purchase, the cars pretty much sell themselves. One vs. another, when you're talking about the bread-and-butter models, the midsize sedans, the family compacts, the comparisons are negligible. The manufacturers know what they're doing and are going to build cars that people want. No problem there.
So what the salesman brings to the experience is himself. If you're really good at making people like you, at making them feel confident about their choices, then you have a good shot in the field. You have to be confident, knowledgeable about the product - you can't just shmooze your way through a sale, you have to know things about the cars and in what ways they're superior to other options, in what ways they're inferior and how you can turn that weakness into either a strength or a non-issue. If it was a simple game of numbers then very quickly only one make and model would ever get sold anywhere. But it isn't a game of comparing specifications, there are emotions involved because you're dealing with people.
Play straight with customers and you'll get a reputation as a good guy to buy a car from. Sell a buttload of cars and you'll get a reputation as the guy to have selling the cars.