AJShorty wrote:
AARG! Thank you for the response John. Is there a way to resolve? Or is it too late?
At best all you can do now is start at Step #6 in the article @John Galt linked where you erase the disk & reinstall macOS, but first make sure you have a good backup of the data.
Restoring that data after the clean install, however, is the hard/tricky part since you don't want to bring back anything related to the old system & user. All you want is to transfer your own personal data files....nothing from the previous owner. It is easy enough to manually select your own personal data files in Desktop, Documents, Download folder to copy it to an external data drive or network share. The tricky part is if you have data in any macOS library files such as Mail, Music/iTunes, Video, Photos apps/Libraries, or if you use cloud based file syncing services. You want to avoid restoring everything if this macOS user account was from the previous owner. How you do this is beyond my ability to provide details here.
I can say that using the third party app Carbon Copy Cloner (CCC) is a good option since you can manually select & deselect files & folders that you want to transfer to another location (or you can even transfer them into a read-only DMG archive file & put this archive DMG file on external media so you can restore its contents later). CCC has a free trial period. You can use CCC to clone the entire home user folder so you can be 100% sure you have everything including the previous owners files/settings so you can take your time making sure you have all of your own personal files before deleting them from the external media and/or DMG archive.
FYI, whenever you receive any used device, it is best to make sure to perform a clean install of the OS even if the previous owner properly prepared it for sale. For Apple computers, that means the new owner of a used Mac should start at Step#6 in the Apple article @John Galt linked. This way you know it was done correctly and it can sometimes reveal whether the previous owner left any firmware locks or is still managing the device.