Can seller lock/erase my second-hand Mac with Find My Mac?
I can confirm (from second-hand bad experience &amp; first-hand testing) that a complete disk erase &amp; clean reinstall does not disable Find My Mac. You need to also either:


Reset the Mac's PRAM (aka NVRAM aka firmware settings). You can do this by holding Command-Option-P-R at startup, and holding them down until the computer has restarted &amp; chimed three times (actually, it's reset by the second chime, but three is traditional).
Set up FMM with your iCloud account. Since the Mac can only be FFM'ed with one iCloud account, this'll replace any previous setup.