Marimo balls are actually a species of algae- cladophora I believe.
It's native to asia, and forms the typical round shape all on it's own. Cladophora only grows spherical in one area of asia, and is a common nuisance algae elsewhere. Some cheap moss balls are actually the nuisance cladophora rolled into a ball, but eventually they fall apart-
By comparison, the real moss balls will always stay round (-ish).. You can even cut them in half, and eventually both balls will form into complete circles again, albeit smaller.