True, for a marketplace platform to be successful, you will need healthy growth of both the consumers & sellers of a service/product.

A good thing is after eBay established itself, there have been several platforms that have been able to successfully grow marketplace platforms - such as Uber/Lyft, ODesk (now Upwork), Etsy, AirBnB. So you can compare and see how they were successful.

For most of the platforms it is usually key to identify the 'demand' & 'supply' side of the marketplace. Make sure there are sufficient listings/service providers for the 'supply' side of the marketplace before you begin targeting or sourcing the 'demand' side.

In the case of Uber it was 'drivers', ODesk - 'freelancers', Etsy - 'Sellers/products'. Once you have sufficient listings on the 'supply' side, then demand will automatically pick up.

Here are a few strategies to help grow the marketplace
- make sure the quality of the listings and the users are high. You should vet your users as they go through the sign up process
- The marketplace should offer a uniquely competitive solution that both type of users find valuable
- Provide customer support and seek feedback from users to ensure you can improve the marketplace experience

Again there are several more areas that should be focused on as the marketplace grows such as Payments, Dispute Resolution etc. I have had the opportunity to build marketplaces from groundup, and would be happy to review your marketplace offering and provide feedback. Feel free to reach out.

cheers