Ask HN: How to solve the cold start problem for a two-sided marketplace?

I'm building a P2P crowdshipping marketplace, basically BlaBlaCar but for packages instead of passengers. Travelers going between cities/countries carry items for people who need to send stuff.

About to launch the MVP and hitting the classic chicken-and-egg problem.

Travelers won't sign up without packages to carry, senders won't post without travelers available. Every marketplace founder says "focus on one side first" but nobody gets specific about how they actually did it, especially when you can't fake supply like you can with a SaaS landing page.

For those who've built P2P platforms or two-sided marketplaces: what actually worked for your first 50-100 transactions? Did you manually match people? Subsidize one side? Constrain to one route/city?