What to do when your selling your Mac book pro
What steps do I need to take if I'm purchasing a used Mac book pro
iMac Pro
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Be extremely careful purchasing a used Mac. Here are the instructions the previous owner should have followed before selling their Mac:
https://support.apple.com/en-us/HT201065
Unless you purchase a refurbished system directly from Apple I highly recommend you perform a clean install of macOS by first erasing the whole physical drive before installing macOS. This will give you a clean system free of any signs of the previous owner. Plus it will make sure the system doesn't have a firmware lock on it and whether the Mac is still being managed by a school or business. Here is an Apple article showing how to erase the whole physical drive:
https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac
Keep in mind that if the Mac ever has a firmware lock that you don't recall the password for, then the Mac will only be able to boot to the internal drive which means you would never be able to boot into Recovery Mode or from a USB drive because Apple requires proof of purchase to remove a firmware lock which you will be unable to satisfy because you purchased a used Mac (unless you purchased a refurbished Mac directly from Apple).