Some call it "chicken and egg problem". Classic of two-sided marketplaces!
- Outsource/"borrow" supply from other people until you can bootstrap the demand side (eg: scrape/syndicate other marketplaces; subcontract services)
- Heavily incent the supply side of the marketplace (eg: offer a guaranteed base pay to subcontractors regardless of demand)
- Bootstrap the supply side yourself (eg: for a forum, use sockpuppet accounts to create the appearance of of a thriving community)
It's hard!
"Hey, I saw your listing on X. I wanted to invite you to list your project on Second Founder. We have X,XXX views per month. You don't have to do anything except create an account, and I'll migrate this listing over."