A lot of older people are too trusting and easily scammed by people like that. Where I live, we get the roofers and driveway paver scams where they paved someone's driveway in the neighborhood and have some leftover material if we're interested. My husband deals with them. I wouldn't even open the door for them.

I don't know how you can get through to your parents except to maybe keep reminding them about how the people that show up at your home promising free stuff, etc., are just scammers looking to take advantage of old people.