Unfortunately we can’t trust most people that we know, and people usually learn this the hard way. When someone trying to sell you something comes to your door, and doesn’t take no for an answer, tries to get indoors, and looks very interested to see what you have inside the house, it is time to not back up and not open the door any more than you need.
For these situations it is a good idea to have some security equipment at hand, which isn’t intended for surveillance. Having a deadbolt on your door will help prevent someone coming in without keys, but having a safety chain will help you keep the door open only a little to see what the seller wants. You don’t have to open the whole door to see and hear their sales speech, and it isn’t rude to not let them inside the house if you don’t know them.
Be especially careful in situations where you ask the person to leave, and they won’t seem to take no for an answer. In these cases it might be a good idea to tell them that you don’t want what they are selling, and that you will call the police if they don’t leave your premises. Close the door and go to the phone to wait and see what they will do next. Most of the time they will go away, and they were just very pushy salesmen hoping to get commissions, but there are situations where they aren’t actually selling anything, but they are just trying to get inside your home and either see what they could come back for later, or make the grab while they are there.
When the situation has passed you should try to discuss this with your neighbors as well. If they have experienced the same situation it could be a good idea to report them to the police just in case they have been doing it elsewhere as well.
Home security is more about preventing than dealing with problems that have already happened. You don’t want to let someone in, then have a gun ready to shoot them if they make a wrong move. The best way to deal with this is to not let the problem arise in the first place!