You could potentially try to get a good amount of products on your marketplace through your friends and family as a start. Also make sure that you go city by city so that way you can focus on one city and once you are doing great there you have the credibility and the experience to move to a new city.

You could also look at other marketplaces like facebook marketplace, craigslist or letgo and duplicate some of the postings on your site (Check for the legality of this). But you can definitely talk to some of the people that are selling on these platforms and try to get them to post on your platform as well.

If you have some investment money behind you, you could buy some stuff off garage sales, craigslist etc and put it for sale on the platform. Then you can advertise the platform to the general public through social media etc.