I have worked with a marketing expert here on clarity and we talked about landing page and focusing on getting the user side signed up. He was very helpful and all makes sense but still not making progress. I hired a marketing person to help with messaging, but still missing something.
So this is going to be a two sided marketplace? That's one of the huge challenges of anything with two sides whether it be an OfferUp style cone or a dating site. You can't get men without women but you can't get women without men or in the case of a selling platform you can't get sellers without buyers but you can't get buyers without sellers, it's kind of a catch 22.
Pretty much everyone from Tinder to Reddit to all other types of sites wrestled with this problem. Most of them used bots to fake activity. Probably didn't provide the best user experience but was at least good enough to lure in some users and from that point it was just organic growth.
Not sure what type of marketplace or site it is but try to get friends and family on there, maybe provide some incentive for signups through an affiliate marketing type situation. I'm actually doing some affiliate stuff for a new cryptocurrency exchange and they are paying $25 per sign-up, that's a little steep, especially for a startup but you could do something on a smaller scale.