Ummm......I don't think most people take a camera with them when they answer the door and considering many of these people work in pairs and while one keeps the homeowner distracted at the door the other is breaking in at the back, asking them to wait and saying "I'll be right back" so you can go grab your camera or phone, really isn't a good idea......just saying
Camera inside on a shelf aimed at the door. There are cell phone apps that allow your cell phone to to act as a remote for your camera. Then there are also security cameras that are motion activated and look like a rock in a flower pot or garden and are not pricey.
Well, that's all well and good if you own a cell phone. Believe it or not, some of us don't.

And many people can't afford or really do not have a need for security cameras. I live in a very rural area and we rarely have anyone come to the door. When someone does come to my door I go out on the porch to them [day time only], I do not let them inside and I keep firmly ahold of the doorknob for a quick retreat if needed. If an occasional driver or misplaced pizza delivery person shows up after dark and knocks for directions, I call thru the door to them, but will not open it.