I founded logotournament.com in 2007 when I didn't even know what a two-sided marketplace was. I only learned many years later that this was considered hard[1]. Luckily, as a self-funded, solo-founder I had no idea it was hard, so I launched my side project anyways. It's done roughly $25,000,000+ USD in sales since.

At the time I was a partner in an ecom firm (few guys and a small warehouse), and I needed a new logo every week or two, so I had the initial demand covered. Acquiring customers is at least 3 orders of magnitude more difficult than acquiring designers.

I launched a private vbulletin forum and invited about two dozen designers from a handful of forums. I posted my first logo contest with a prize of $200 IIRC. I offered $25 each to the first 4-5 people that submitted, to get the ball rolling and building some initial trust. And just like that I had my first successful logo contest. Over a few months I would host a contest 1-2 times a month, and would manually message each designer when there was a new one.

In parallel I started development of the actual site. When it was time to launch, I manually imported the vbulletin logo contests, and kicked things off with a single contest that I was hosting. That site still wasn't a living breathing thing yet, until I had an actual customer that wasn't me. So I burned about $1800 on adwords over a month, and received 3 customers where I made maybe $100 in fees. A rough start, but running nevertheless. After that I was able to get Facebook ads working after lots of trial and error which led to the first 100 customers.

[1] It's not that the actions were hard, it's that a successful result had a low probability.