"...it's really simple" - this can come across as "you guys aren't smart enough to figure this out?"

A door is used for more than solicitation, but salesmen exploit that to get in front of people, many of whom aren't interested in being solicited, and have even put signs up. The attitude that they just shouldn't open overlooks the burden of having to check who is at the door because the salesman has initiated the cultural norm of knocking or ringing a doorbell.

So coming from a disliked industry, telling people it's simple, and then how they should behave to save themselves and you(!) time rubs people the wrong way.