How did you solve the cold start problem for a two-sided marketplace?

We launched a marketplace (nairel.com) for buying/selling with AI listing tools. Platform works, ~400 products, but getting the supply side going is the real challenge.

We've tried: cold email outreach to independent retailers, SEO/blog content, social media. Cold email has been the most effective so far.

What worked for others who've built marketplaces or platforms with a supply/demand problem?