Have you ever tried Flohmarkt? Flohmarkt is a decentralized app for buying, selling, and bartering in the Fediverse that allows you to post items for sale or to give away. Because it's federated, users of other platforms (e.g., Mastodon) can interact, view listings, and contact sellers directly via private messages.
https://wedistribute.org/2024/08/flohmarkt-federated-market/