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.

Acquiring and retaining users requires two different things. Recruiting users: connect with groups. University students can use cash upon graduation, so selling still usable furnishings, clothes, decor, etc. will appeal to them. An accessible subset of university students is a sorority or fraternity. Connect with these organizations (undergrads make great brand ambassadors for this purpose in exchange for using your platform fee-free). You should take a transaction fee from each sale. Expand awareness of your platform via the sorority and fraternity regional and national chapters. As these early users age, their social networks change which creates user diversity. I specialize in identifying, developing and executing growth channels and I'm just a phone call away.

I would say this from my personal experience.

BREAKING DOWN 'Customer To Customer (C2C)'
At its most basic level, C2C represents a market environment where one customer purchases goods from another customer using a third-party business or platform to facilitate the transaction. C2C businesses are a new type of model that has emerged with ecommerce technology and the sharing economy. The advantage for customers is that they benefit from the competition for products and they can often find items that are difficult to find elsewhere. In addition, margins are high for sellers because there are minimal costs due to the absence of retailers or wholesalers. C2C sites are convenient because there is no need to visit a brick-and-mortar store. Consumers simply list their products online, and the buyers come to them. In the case of eBay, items are shipped by the seller directly to the buyer.

Revenue and Growth of the C2C Market
C2C websites and similar platforms make money from fees charged to sellers for listing items for sale, adding on promotional features and facilitating credit card transactions. These C2C transactions typically involve products sold through a classified or auction system, and the products sold are often used.

The C2C market is projected to grow in the future because of its cost-effectiveness. The cost of using third parties is declining, and the amount of products for sale by consumers is steadily rising. Retailers consider it to be an important business model because of the popularity of social media and other online channels. These channels showcase specific products already owned by consumers and increase demand, which drives increased online traffic to C2C platforms.

However, C2C has some problems, for example, lack of quality control or payment guarantees. Is some cases, credit card payments may not be supported although the emergence of PayPal and other payment systems over the years has helped to simplify payments on C2C platforms.

Example of the Rise of C2C
The C2C marketplace has increased over time as more companies have entered the space to facilitate C2C transactions. Many companies target niche markets and list specific products to attract unique consumers.

For example, Amit Lakhotia, former vice president of payments at Paytm, left his position in January 2016 to pursue other ventures, one of which was Tokopedia, one of Indonesia's biggest online marketplaces. Tokopedia is a C2C retailer that provides a platform on which entrepreneurs can open small and midsize C2C enterprises (SME) of their own for free.

If you have any other query you can consult me.

Launching a Consumer-To-Consumer marketplace can be challenging, especially when it comes to attracting the first users. Offering initial incentives for early adopters, partnering with local communities, or hosting promotional events can help drive interest and engagement. Also, you might find this article helpful for insights and strategies: https://www.cleveroad.com/blog/how-to-make-an-app-like-letgo/. Good luck with your launch!