To grow a marketplace, it is usually about the hustle in contacting every potential supplier and consumer individually.
I'll focus on the consumer side of the equation since you already have the supplier part sorted.
Instead of posting about your website, contact every business that is looking for designers online, be it on Craigslist, forums or freelance websites. It's all about the hustle. A marketplace like yours is usually more appealing than the platforms above since a business can pick and choose the design they like most and pay for it.
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