You understand enough of it to question the nature of this market place.
In most cases, people who are about to launch their own market place don't talk about ROI or allowing you to make money. They focus on how to market themselves, attract traffic, compete with other market place sites, etc. The reality is that the profit margins of existing market place sites are so tiny, that there is barely anything to gain for investors.
My personal opinion is that a market place site should be simple without too many bells and whistles. Plenty enough options to choose from nowadays.