There's really no way to tell what kind of conditions the rabbit is living in just from that photo, even with a rusty cage. It could just be its home base, it could have a separate run attached. And just from the photo, it does appear to likely be healthy. But again, no way to tell for sure from a photo.
It's unlikely Mercari allows the sale of animals on their site. If you have concerns, I would suggest contacting Mercari to report the posting. If you don't feel it's enough, if any laws were broken, you could report it to animal control in the applicable state. Or see if a local rabbit rescue would be interested in helping the rabbit.
I doubt hostility was intended by the above RO members post, just a misunderstanding. We get a lot of spammers popping up here, and it can sometimes be difficult to tell if a first post is genuine or spam.
