You are buying 1 GIGANTIC, beautiful Marimo Ball or commonly called Moss Ball. They will be about the size of a Tennis Ball which makes it very rare and big. Between 2 1/4" to 2 1/2".
Moss Balls are a very easy plant. Just put it where you want it and watch it grow. All it requires is to be turned every now and then to prevent the side that is touching the substrate from turning brown. It's that easy.
In Japan Marimo Balls are said to bring good luck. ;-)
One of the pictures to the right shows the gigantic mossball next to an average 1" mossball. This gives you an idea how big and rare these are.