The old name was Bouncer. These were the security guards of years gone by. The Term Door person is now someone licenced (in the UK particularly) to be a security person on the access door to clubs, pubs and social gatherings.
a door to door sale person
a salesman
The person who lives next door to you is called your neighbor(s).
The person who opens the door in traffic, The reason why is when anyone exits their vehicle they should always look and be sure it is clear and safe to do so. If a person does open their door in traffic and causes an accident regardless of whether the door is damaged or not that person is liable for damages, injuries to property and people as a result of negligence.
The implication of 'at the door' is that a person is waiting to be given access or at least to talk with someone who is at home. It would usually be a person who is 'at the door'. "The repair man is at the door", "John and Mary are at the door." 'By the door' could refer usually to any object that happens to be near the door, inside or outside. "Where is mom's umbrella?" "It's by the front door".
A solicitor? Possibly? Aren't they called a salesperson?
Bellhop
The light reflected from the person is absorbed by the door. So there's none left on your side to enter your eyes and form an image of the person, or of anything else on his side of the door.
The person that opens the door is at fault
The website Adzuna lists the average salary for a door to door sales person in Canada as $37,672. This is 23% lower than the average Canadian salary.
You have to get Hercules
When a door is opened, mechanical energy is transferred from the person opening the door to the door itself. The person exerts a force to overcome the resistance of the door, causing it to move and change its position. This energy transfer is an example of mechanical work being done.