- 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!

I like the bootstrapping idea... github projects may appreciate getting buyout offers out of the clear blue! And, submitted offers could be fed back into the valuation model for fine-tuning.

In this example, the OP could reach out to everyone who has listed a project on Micro Acquire, Flippa, and similar businesses acquisition marketplaces.

"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."

More like: “we have already copied your listing for you and it is live here: ... let us know your contact details and we will as them!”

The solution is posting this marketplace on a forum full of people who might like to buy and sell side projects