Am I being rude?

Of course I am.
What is the protocol when someone drives up while you are working on your car?

I am working on the front of my car and the trunk is toward the street. People love to drive up and want to talk to you about your car. Why do this "people" expect you to stop what you are doing and walk down to the street, to talk with them.
I personally think that if they want a conversation they should get their *** out of their car and walk up to start a conversation.
Although, there was the time, a guy walks up to the car as I am underneath it and says hello. No I am thinking, it must be a looky lou , so I half yell, Get the **** out of here. Only to glance out and see a uniform pair of pants. It was the Postman. I did apologize, but he was nice about it.

Usually the arabs and gypsys will drive up and yell, How much do you want for it. I usually yell down 50 or $75k. One older guy sat there thinking about the price, I kinda felt sorry for him, so I told him the car was not worth that much