Open a saved item and use Share to Facebook. It publishes a listing for that item on AntiquesDirectory (a sister site) and opens a Facebook post. It's listing only — there's no online checkout; sales still happen in the store.
Once shared, the item shows a Remove from AntiquesDirectory button if you want to take the listing down. Items also come off automatically when they sell.
Your store must be linked to an AntiquesDirectory page first (an admin sets that up). If it isn't linked yet, the button will tell you.