Given where you live, I can understand the concern. In my area, the scam in the summer was to just walk into the house. People leave their door open while they are gardening, working in the yard, etc. We put in a combo lock on my parent's front door. That way it is always locked, yet immediate family members can enter.
Someone coming to ask if you want to sell an old car is not actually that unusual. My late Dad was approached I can't count how many times about selling his 64 T-bird. I was approached at least 3 times for a 93 Mercury Marquis. Very often, if the body is crap, they want a parts car OR want to part out a car (for a lot more money than they give you). That's fine - not a business I want to be in. My vehicles last a long time because of junk yard parts!