Showcase has INLINE moderation that allows bulk deleting of content. That is the preferred way as the DataWriter handles post delete methods like cleaning up all of the XenForo integration (newsfeeds, alerts, likes, reports, warnings, logs etc). Manually deleting would orphan a bunch of stuff if you don't know what you are doing as far as cleaning up records in core xf tables added via handlers.
If its not a lot of content, I'd recommend doing it via inline moderation and then running the Showcase Rebuilds after you are done.
There is also the Uninstall and then fresh install method (altho, this wouldn't work if you are just wanting to delete ITEMS and retain Admin CP stuff like Categories, Prefixes, Item Fields, Review Fields etc).
I'd recommend backing things up if you decide to manually delete stuff without letting the DataWriter(s) handle post delete functions.