- you're on HN, so you have an opportunity to tell us the name of the thing - marketing
- perhaps consider your marketing budget to be the area you need to invest in now, and indirectly how that budget can actually generate a little revenue and exercise the engine - e.g. do you have packages you can ship through your service to announce it to others? Use your marketing budget to do it and collect your marketplace fees - marketing again. Nothing says I believe in my product like using it for real (IMHO).
- a bold and risky move (?) - most would say fake it till you make it - but I for one tire of this tactic. How about approaching this with honesty and reward the first adopters? "We're brand new but you can be the first to help us prove out this model?".
- your marketplace is a network. Search for an opening that has viral properties. Try to tap into something that has a network effect, go to where your customer is and see how you can target/advertise strategically and respectfully - become a trusted partner to one or more communities (e.g. thinking out loud, eBay sellers maybe?). This could include finding the right partner(s) who have a problem and are willing to give you a shot in an existing network.
On the last point - as an example of the viral thing - I worked on a real estate tool a while back. We found a viral hook - there were for sure properties that needed to be processed and worked through the tool - we email invited the parties involved in the transaction to invite them to work on the property in the secure tool, and we gave them the ability to invite others working the same transaction to the tool, and we focused everything on polishing that workflow and experience.
This way, as soon as one person used the tool they could invite others to use it legitimately to work in the tool and that was the viral aspect.
This points to, replace real estate tool and house with "package" and invite... and can you achieve something viral that spreads itself... like, when someone ships with your tool, it emails the recipient with the link to your site for status tracking and a call to action to make them want to ship using your platform.
This to me is a lot of marketing and product strategy around incentivizing the network effect.
Disclaimer: I never made millions of dollars off a marketplace. But I did help stand up the real estate mechanism I mentioned and that business reliably brought in 5-10 grand a month with no marketing effort and just that one mechanism, and it also helped us find a few key network partners. That's what drives my feedback.