Flohmarkt is a free open platform for small ads and selling things ("Flohmarkt" is the German for flea market). It's part of the Fediverse so the small ads can federate to other servers, and people can interact with Flohmarkt from Mastodon etc accounts.
If you want to post an advert there is a server list:
https://codeberg.org/flohmarkt/flohmarkt/wiki/flohmarkt-instances
There is technical info on their Codeberg page:
https://codeberg.org/flohmarkt/flohmarkt
You can follow their lead dev's account at: