I have used Instant Passive System. Usually as a solo ad seller you need to pay for high end coaching services to learn the skills for this method. But this a very cheap and well-explained course.
The voice of the trainer inside this course is bit low. May be you need to use a headphone. This is the only con I have found in it.
Lastly, I would like to say,- Instant Passive System is Strongly Recommended for Newbie Solo Ads Seller.