The only sure way i know of protecting the previous owner's privacy is to install a different hard drive that's been wiped & tested, and then do a OEM install of the operating system. It depends on which OS it is .... but i think they're available from Mac for a pretty reasonable price. It's a small investment, but then you know the job is done right. It's also the simplest and easiest way too.
Keep the original drive on hand for a little while. Once you know you won't need it anymore you can either wipe or destroy it.( If opt to wipe, make sure to nuke any hidden GPT partitions that might be on the drive as well.)
Edited to add: It's just an opinion, but the re-install options are nice when you've got a problem with the operating system that you don't want to spend hours (or days)on. They're not so great for data security.You can tell because a fresh OEM install will take up a certain amount of space on the drive. A reinstall will often be two or three times that size. It's got to be carrying a lot of data over from the old to the new.
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