Does the Wii U you actually has Homebrew (Haxchi, ect.) ?

 

It doesnt really matter if this user is on it or not, you can still add new users.
other than that you could try and use a file transfer program like filezilla to delete the user via nand, I wouldn't recommend that though, one small mistake and you're doomed.
formatting the system is also an option.
Ask the seller to change the password via the nintendo website to something simple.

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We're not talking about parental codes.