The trick is that the audit link only appears when you sign up for the individual course, not the entire sequence. So if you go to this link:
... and click "Enroll", you can only proceed by supplying payment info. However, if you scroll down to that page to the box titled "Course 1", at the bottom of that box is a link "You can choose to take this course only. Learn More".
Click on THAT to go to the individual course page. Then, click Enroll, and in the first box that pops up, you'll see the link "Or audit this course" in the lower left.
This allowed me to sign up for all five without supplying payment info.
When you audit a course:
- You'll be able to see most of the course materials for free, but you won't be able to submit certain assignments or get grades for your work.
- You won't be able to submit assignments for feedback or a grade.
- You won't get a Course Certificate.
So now they've effectively eliminated just about the only thing that distinguishes them from some YouTube videos and a textbook.
The EdX solution to this is that their courses are all endorsed or run by brick-and-mortar universities with considerable investments in their brand that they won't want to tarnish by attaching it to any random certificate.