Sign a few sellers, and give really good offers for them to use your site. Then help those sellers advertise. Do it again. & Again Keep building the snowball u

Sign a few sellers, and give really good offers for them to use your site. Then help those sellers advertise. Do it again. & Again Keep building the snowball until it's big. It's not easy, but that's one possible one. It should much much easier nowadays with all the social networks, but nothing is easy. Just a lot of hard work and convincing.