I have always wondered about that model when it's not vertical and anyone can sell anything Digital, Physical or Service. (Gumroad, Selify, etc)

Marketplaces tend gain traction faster when they are vertical and specific. My best word of advice would be to select the vertical that's bringing in the most traffic, (Hope it's a big vertical in itself) and focus on studying the consumers of that vertical and BE THE ONE ECOM SOLUTION for that vertical beating everyone else hands down (case in point AirBnB vs Craigslist). Make the experience of the specific vertical buyer better than any other marketplace. Focus on making the experience flawless and friction-less.

It'll become much easier to add other verticals later vs. trying to be everything for everyone from the get go.