Hello - this is a classic Chicken-Egg question.

First, you need to identify the specific need that the buyers and sellers are facing. Are there already marketplaces that do what you're suggesting? Do those marketplaces have weaknesses? Do they have massive brand awareness that would make it difficult to overthrow?

If you have solutions to these questions or challenges, I suggest first targeting the creatives. Make it dead simple for them to create a profile and explain that you're building your marketplace. Perhaps you can incentivize them with waived fees, exclusive options, or "Founding Member" benefits.

Once you have the community of creatives, start matching the specific needs of the client with the creative, perhaps running Facebook ads to new founders advertising branding services (which in turn takes them to the "Branding" category of your marketplace).

Then, start building your content library to attract organic traffic. Focus solely on the specific needs of the client in one blog and the specific needs of the creative on another. Position yourself as a partner, not a vendor. You are there to support their success.

Good luck,
-Shaun