We want to launch a Consumer-To-Consumer marketplace like Letgo or Avito. How could we acquire/retain the first users without publishing fake products?
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.