Need help finding plugin!

Hi!

I need help. So i want to create a public site where people are posting avertisements on what they are selling or if they want to exhcange something or sell something. Just like the idea this site here has – ss

I have looked all over the web, searched forum, but i dont even know for what kind of terms really to look for.

Maybe somebody knows this, i would be really glad if you could share!