- Start in a niche where you can lean on your own personal connections to kickstart the demand/supply side. Once you've established yourself as the best platform in that niche, go broader.

- 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!