Do not open the door for door-to-door sales people. Badges are easy to get or make. Even legitimate badges don't necessarily guarantee that the guy has had a background check and actually work for the company. Independent contractors have legitimate badges; Comcast is a prime example.
Second - carry at home. A little pea shooter will do. Gives you an extra few seconds to get to a real gun.
I will disagree with calling 911 first. Grab a gun first, 'cause you may not get a second chance if someone is trying to break in. 911 is #2.
"With great power, there must also come great responsibility." - Stan Lee
"All that is necessary for evil to triumph is for good men to do nothing." - Edmund Burke