First of all, congrats on starting your business! I was head of product for a marketplace company in the hiring space for 4 years so I have a significant amount of experience building a marketplace similar to yours from the ground up. I also know how challenging it can be to reach your customer base! These types of businesses are extremely fragmented and so it can be difficult to reach "critical mass" where your cost per new customer drops significantly. Here are some suggestions:

1. Study your competition. There are plenty of other marketplaces out there like yours (at least given the brief synopsis above) so you can see what has worked for them by studying their public growth tactics. Are they relying heavily on Google AdWords? Do they put out a lot of content that gets significant traffic? Do they have strong referral programs/rewards that seem to be successful? Take a look at these and assume that anything they are doing is at least worth a test for you

2. With any marketplace it's incredibly important to grow both sides of the market (supply AND demand) at similar rates, otherwise one side will not be happy (and if they aren't happy they won't retain). Because of this, I would be hyper focused on your growth by either field/location/etc. Don't try and build your entire platform across all categories at once. Start small with one or just a few key, high demand, categories and focus all of your marketing budget/attention at these demos. If you spread yourself too thin no one will be happy.

3. Rely on what I call the "second mover advantage". Because there are other platforms doing similar things, find ways to use those platforms to grow your own. Contact freelancers on the other platforms, or post your own projects that eventually lead people back to your platform. Make sure you aren't violating terms of service, but do everything right up until that line to bring customers from more established platforms over to yours: offer incentives, discounts, even operate at a loss to get your first customers on board. You are lucky that you don't need to build demand in this sector from scratch so use that as an advantage.

Good luck, and if you'd like to discuss further let me know!