With iCloud, the App Store, Find My Mac and everything else that Apple has designed into the OS to make stealing a Mac difficult, they have also made buying a Mac difficult.
All of the apps on that Mac belong to someone else, as far as Apple is concerned. The operating system as well. If I were you I would download High Sierra and use a program like DiskmakerX to make a bootable USB thumb drive installer.
I would back up all your documents. Everything you need from that Mac and then boot from the thumb drive and, using Disk Utility in the installer, reformat the hard drive so nothing of the previous owner is left on the computer.
You can remove apps, you can change the name, you can hope he disabled Find My Mac, but for me all of that is death my a thousand cuts. It's easier, in the long run, to wipe his footprint from the computer by doing the format and reinstall then endlessly fiddling with all of the myriad ways you will have to work around what he has already done with the Mac, IMHO...