Delete user account - High Sierra - 10.13.2

I have, since I bought my device second hand, an account that was created by the seller. I want to remove it but can't. I have removed the admin capabilities of this account but still the "-" in the account menu is unavailable.

I have logged in this account and closed the session to be sure, doesn't change a thing

You can see the lock is open...

Do you know this problem ?

You need to wipe the Mac & start over.

Right now you don't own even the OS, let alone any of the other software on the Mac. You will at some point come up against the issue that an update requires the Apple ID of the original owner & you will then be stuck.

See Apple KB - What to do before you sell, give away, or trade in your Mac
The original owner should have done this. As they didn't, you need to do the last one - Erase your hard drive and reinstall macOS.

By wiping the Mac & starting afresh with your own Apple ID, you will then have full control over the Mac.

Depending on precisely which Mac it is [if it is new enough to have a T2 chip*] you may need the original owner to retrospectively do the first 6 steps or you may be locked out. If you reach that point & cannot trace the original owner, then only Apple can unlock it & will require proof of ownership.

*Apple KB - About the Apple T2 Security Chip