Flohmarkt is federated decentralized classified ad software using ActivityPub. They showed off how you can use other fedi platforms (e.g. Mastodon) to interact with small ads and talk to the seller via direct messages.
Demo by Philip Patsch at the most recent #FediForum.
https://spectra.video/w/5GXnaWQ4BByS55J8hM7EpB
/cc @Profpatsch
