When i go out doing stuff like this I usually wait to stop in untill i see them outside on there lawn, or in there garage/driveway tinkering. Then walk up and come right out and say that im interested in the car/truck sitting out back.
I even have my own outfit for these occasions too. Blue jeans, plain jane t-shirt or flanel, some work boots. If your gonna wear a hat dont wear anything stupid, and wear it like hats are meant to be worn. Not sideways, backwards, or with upside down brims.
Its all about respect and politeness when your talking to people you dont know. Especially when you want something from them. haha.