Someone else's Macbook Air showing up under my devices in iCloud?

[ravenvii]

So for me my old iPad 1 appears twice in the device list in my find my iPhone because I set it up as a new device at some point without deleting it from iCloud. In that case it would be impossible to do what you suggested. In addition it is not online ever. I sold it to target for the upgrade voucher so I assume it never got sold to somebody who actually uses it.

My post only concerns Macs.

For iOS devices, it's much simpler. As long as you Erase All Contents and Settings, you're fine. Just remove the device from your Devices list (hit the little x on iCloud.com, or swipe and tap Remove in the Find My iPhone app).