Why do you think they are selling their courses or Trying to scam you . If the Course they are telling is paid then it make sense and if it is not paid and it available on YouTube then they are just trying to help you maybe the technique that worked on them , may also work on you and i have also suggested so many people in this reddit to watch yt videos or free workshop(50 min video) of Speakyourmindmethod he helped me a lot to confront my stutter and get better