hi Joe,
So, I used to own the 4th largest freelancer marketplace 5 years ago, there are many challenges. You have mentioned few type of marketplaces, each has its own challenges. I have moved to other business since then ( i ended up selling it as i received an offer from the 2nd largest competitor at the time ).
My largest struggle after getting it successfully up and running, wasn't maintain the sales , SEO, marketing , etc but really keeping scammers off the site, there is a big portion of scammers that target these sites and one of the main reason a lot of those businesses end up out of business.
There are many solutions out there help you prevent those scammers and keep them out of ur site, but they tend to cost money ( i currently using a security solutions of all type for my business, email verification, phone verification, etc.
I would encourage you to study the market pretty well before entering, i spend 3 months just studying my competitors before i jumped in because i was aware it is a tricky market and more competitive than many people think.
We always look at the surface, and surface always looks good, it is what is underneath that should be of a real concern.