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.

 

The tool to remove a user account without the password is called account manager (don't think can link to it on here).

Of course the Wii u would need to be in a state to run homebrew for this to work

 

I had 3 different nnids and not 1 pin, i clicked forgot code and then visited this similar site https://mkey.salthax.org

1. On the Wii U Menu, select the Mii character in the upper-left corner to access the "User Settings."
2. Tap "Delete This User."
   • If this selection is grey, there is only one account on the system, and it cannot be deleted. You will need to format the system instead. How to
3. If prompted, enter the four-digit PIN and select "OK."
4. Enter the password associated to the Nintendo Network ID, then select "OK." (Forgot Password?)
5. The nickname and Nintendo Network ID associated to the account will display. Select "Next" to continue.
6. Select "Understood" to agree to delete the save data and play times associated with the user. Select "Next" to continue.
7. Do you want to delete just the user account (Mii, save data) or the user account ANDNintendo Network ID?
   • PLEASE NOTE -
     • Deleting the user account will delete the save data, user Mii, and play records associated to the account, but will not delete the Nintendo Network account. The Nintendo Network ID can be relinked to the same console in the future. How to
     • Deleting the Nintendo Network ID will also delete all Nintendo eShop licenses and balances on both the Wii U AND Nintendo 3DS family systems.
     • Deleted Nintendo Network accounts cannot be recovered. This includes ALL purchases or funds lost when the account is deleted.
     • A deleted Nintendo Network ID cannot be used again to create a new account. Once it is deleted, that username is gone.
     • The Nintendo Network ID should only be deleted if you do not want to use it or access purchases made with it (on Wii U or Nintendo 3DS) ever again.
     • Even if you are no longer planning to use the Nintendo Network account, it is strongly recommended that you delete just the user account. That will keep your Nintendo Network ID (and related purchases) available to you to use again in the future.