I have worked with a marketing expert here on clarity and we talked about landing page and focusing on getting the user side signed up. He was very helpful and all makes sense but still not making progress. I hired a marketing person to help with messaging, but still missing something.
As I'm sure you know, two-sided platforms are notoriously difficult to build.
I've built a niche graphic design marketplace site of my own with a few thousand buyers serviced by over a hundred designers. Rather than building up both sides initially, we worked on building the buyer side, with a team of in-house contractors.
Once we had a good pool of buyers, we then worked to recruit designers, knowing they'd be enticed by our active buyers that were already spending money.
This strategy won't work for all marketplaces and I don't know much about your business or the problems you're facing. However, I'd recommend checking out the book Platform Revolution.
It's full of examples of businesses that have developed platforms and marketplaces using a variety of different strategies.