Like the title said, Are Blue Haunted Mansion Rocking Chairs Rare?

And if they are, what could I get for one?

Not so much. They were given out as quest prizes on a few quests. Not sure what one would be worth, maybe someone else can answer that part.

Nope, not rare. Not worth much. They look nice in rooms, though.

what about green rocking chairs?

The green chair are buyable.
To see more mansion chairs, check out the wiki (http://www.disneyonlineworlds.com)