As far as I know, this does not exist yet.
You can usually backup a course from one Moodle system, and upload it in another.
What about a place where people could give away, or even sell, Moodle courses?
Please see Moodle.net and archive.moodle.net.
The Moodle Association of Japan has been trying to excite users about this too with various levels of success. Their site is here
https://showcase.moodlejapan.org/
https://showcase.moodlejapan.org/
Look for open educational resources (OER).