Door to door or door-to-door

Answer 1

It depends on how you are using the words.

If you say, "He sold the books door to door" you mean he sold them ffrom door to door. He is then a door-to-door salesman. The hyphens are put in because the words then become a compound adjective, describing the sort of salesman he is.

It's the same construction as every day and everyday. If he sold the books every day, then the selling was an everyday event for him.

Answer 2

Both forms are correct, but the hyphenated version "door-to-door" is more commonly used to describe activities that involve visiting each house or building in a line.