Another reading/purpose of the Bible salesman is to continue in the tradition of the Gothic theme. O'Connor represents an amazing writer of the Southern Gothic genre. Within the Gothic genre, it is traditional that there is someone who is perceived as pure, religious or righteous. This role is typically filed by a nun, monk, or other institutionalized religious figure. This person then must go against what the reader believes them to be: honest, compassionate, religious. In O'Connor's story the Bible salesman should be right and fair, like the Book he is selling, however he commits a serious crime, stealing a personal object. So he also is used as a traditional character within the Gothic genre O'Connor is writing in.
The Bible salesman is the foil to Joy/Hulga. Joy/Hulga is so aggravated with the stupidity of the "good country people" who surround her mother and herself. Joy/Hulga has college degrees and considers herself so much further above these people...they are so stupid and uneducated...that she never even sees the salesman coming. He is cunning and slick. His goal is to get what he can from the young ladies he targets. He has several "trophies" in his salesman's case, along with cards and a flask of alcohol (all of which is ironic in the case of a BIBLE salesman...supposedly a religious individual). He gets J/H in the hayloft and gets her to take off her leg, which he promptly takes off with, never to be seen again. He is her "foil" because he brings out the best in her, and also shows the audience that he is every bit as onery and hateful as J/H has been acting throughout the story before his arrival.
This is just the event that J/H needs to humble her a little and get her to be a little less hateful and more accepting of the people in her community.