If you are building a marketplace.

If you are building a marketplace and start from making your platform, you are starting at the wrong end.

The triumphal march of AirBnB began with placing their ads on Craiglist. Only after analyzing their and other people’s ads, they realized that the most important thing that increases the number of reservations is beautiful photographs, and they began to invest money and energy in photographing.

Musicians become famous because they post their videos on YouTube, not because they start promoting their website.

If your offer does not stand out among other ads, then its no better than others. And in this case, building a separate marketplace will not help you. 

Almost every niche has its own craigslist or youtube. It could be another website or a group on facebook or something else. Start there and test your assumptions.