There is NO reason to ever conduct business with ANYONE who who ever approaches you for any reason. If what they are selling is interesting take info and work backwards thru legit websites or phone lines that are widely advertised.
Hmm, I don't know about that. Besides giving away the watchtower, I have had folks come to my door offering a service. Usually they are a smaller company doing work in the neighborhood and looking to drum up business. I have had tasks like tree removal and driveway seal coating done by these type of contacts with great success. While they were too small to have "Websites or 1-800 numbers" they offered local references for their work.
I must be old school because I tend to put faith in talking man to man with someone, looking in their eyes when I ask a question then judging for myself. There are a billion websites out there and most are as big a fraud as any human can be, so that doesn't mean a company is legit either.
As a society, If we don't answer our doors to the world because we are afraid or skeptical of everyone who would ever knock, we are living in fear.