I'm currently adding the price and location data to the post in the streams repository (since we provide distance search and other location services). Suggestion: It might make sense going forward to use the standard ActivityPub location (Place) schema for this and put it on the activity instead of inside a custom element with non-standard attributes. We support that already and then the only special thing I would need to extract from the flohmarkt data element is the price.

What's required to provide a remote interact button? We do support remote interactions in streams via the ostatus webfinger follow template (similar to Mastodon) so this shouldn't be too difficult to add.

Cheers.