The beauty of a "two-sided" marketplace. I can actually tell you how we started ours. I actually wrote a post, but in essence we "seeded" one side of the marketplace with our own video courses. This built confidence "enough" to the point other authors were "enticed" to upload content to our site. Today we have more 500,000 registered students and receive about 1M uniques per month.

Glad to hop on a call and dig deeper or just read my post on how we started: http://blog.tareasplus.com/startup-leveraged-youtube-create-marketplace-500000-subscribers/