Marketplace founders, what's one strategy that convinced providers to list on your marketplace?

Share the approach you took, to developing those early stage relationships with providers (sellers) during the first 12 - 18 months of seeding your marketplace with vendors. How you encouraged them to join the seeding stages, to enable you to have an existing providers database, with items available to purchase before beta stage testing. Don't hesitate to also discuss your failures, and the hurdles you came across during this journey too. Before realising this strategy.