How to restore a used iPad without access to the previous account?

Hello~ You can’t. If it’s activation locked it will remain so unless the account owner who set it up can be contacted.

Take a look here…

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also…

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~

You don't without the original owner removing it from their account. If you could, that would mean that anyone could steal an Apple Device and set it up as their own. It is Activation Lock that prevents that from happening and puts the original owner who set the device up in control of the device until they choose to remove it from their account. A device that is Activation Locked has no value.

You’re kind of stuck unless the previous owner removes the account.

Hello again ~ I am glad you are able to contact the previous owner as most can’t. Also take a look at here…

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

~Katana-San~