Sometimes it is much easier for a writer to simply sell his/her completed novel to another author who is willing to buy ghostwritten novels. It usually has to do with preferring not to have to do marketing (by indie publishing) or simply not having the time to wait for responses from agents and so on (if traditional). There are indie authors out there who have established platforms and are open to buying ghostwritten work from novel writers. But are there sites where prospective ghostwriters can reach such people without simply asking around on facebook and other social media sites?
I'm not exactly sure: do you want to write first and then try to find someone?
But ghostwriting is more about answering the need of someone, so it's the opposite.
What you are requesting is closer from traditional publishing than ghostwriting: I write and then I sell.
I don't see any platform doing what you request; if you're searching for freelance platforms for ghostwriters then try Upwork and those kinds of websites.