If you didn't know what you bought, let me fill you in. You bought an Arri single side follow focus--otherwise know at the LFF-1 I believe. From what I've seen, $700 USD is a very good price for what should be a great piece of equipment (if the previous owners haven't wreaked it.) It will probably need 15mm spaced rods to mount on, and the lens gear (the part that meshes with the lens) looks like it's film pitch. I'm not sure exactly what camera you are planning on using it with, but unless you are using a cine lens, you will need a different pitch gear.
Looks like you got a great deal if you can use the gear.