Is it illegal to video tape a salesman in your home without there consent for the prepose of getting information to sell to another company?

It is if it's ruled that the salesman was under a reasonable expectation of privacy in the home. If the video has audio, it could be a violation of your state's wiretapping laws if you did not have his or her consent. This could be a felony, depending on the circumstances, so the person videotaping could be subject to criminal and civil action.