You need the Original System Discs that came with that system. If they were not supplied by the seller then you will need to order them from Apple.
That system came with either Leopard or Snow Leopard and any other version of OS X that may now be installed is tied to the sellers Apple ID and you can not, and Should Not, use that.
the seller should have restored it to factory settings before they sold it to you. Also, you need the two disks that originally came with the mac, the disk with OS X and the other disk that has the applications on it. If you don't have them, order them from Apple before making any attempt at restoring.