I am building a marketplace. The buyers are women and the sellers are small brands selling clothing, jewelry, and accessories.
Not sure how to tackle the problem.
In most cases, I think it is easier to work on the demand side first by setting up a service that gives the future buyers some free service. For example, in your case, you likely are trying to target women who are into fashion and like to buy jewelry, clothing, etc. So, find out something you can build for that audience for free like online reviews, a widget that let's them virtually "try on" the jewelry, etc. Then once you have something that your target customers are interested in, you can start working on the supply side.
