Second hand Macbook and Find My Mac
Computers, phones, and devices that are not currently logged in with a given Apple ID and specifically with the service Find My Mac / Find My iPhone enabled will show up as "Offline" in someone's iCloud account etc.

So, basically, if both of you have the same computer in your iCloud accounts with Find My Mac, only the person who is currently logged in can track it. If neither of you are logged in no one can track it. The only way he could track it at this point is if you knew his Apple ID and password and went into System Preferences > iCloud and set-up Find My Mac for him. It is up to him if he wants to keep a line in his iCloud.com menu under Find My Mac that has the "name" of the computer that was retrieved the last time he tracked it that will perpetually search and always fail to find your computer, and just say "Offline". You can't change that for him. 

You're safe. With the computer wiped, his user account isn't present, and that's where the iCloud account data is stored. You do not need to track your computer to "block" him from doing. It's just up to you if you want to track it at all for yourself. He will never know where it is again.