What if the previous owner doesn’t know his Apple ID or his password ?

Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be turned off:

https://support.apple.com/en-us/HT201365

https://support.apple.com/en-gb/HT201441

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, or unless you can secure the cooperation of the previous owner, you’ll not be able to defeat or bypass the Activation Lock. Without documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple won’t assist - and nobody here will be able to help you.

If you are the owner of the AppleID that previously activated the device - and you have forgotten your AppleID or associated password, these can be recovered here:

https://iforgot.apple.com

To recover your credentials, you’ll need access to one of the following:

• Your primary email address mailbox that corresponds with your AppleID
• Any of the secondary/recovery email addresses that should be configured for your AppleID account
• Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID

Unless you AppleID account has not been fully/correctly configured, or has been seriously neglected, recovery of the AppleID and password should not be difficult.

More information about recovery of your AppleID:

https://support.apple.com/en-us/HT201354

More information about recovery of your AppleID password:

https://support.apple.com/en-us/HT201487

When you recover access to your credentials, you would be well advised to log-in to your AppleID account from a web browser - and verify/update any email addresses and trusted telephone numbers:

https://appleid.apple.com

If you can't get a hold of the seller, you have likely been scammed. There is nothing you can do without their help removing Activation Lock. Apple won't help you and we can't help you. You are now the proud owner of a useless paperweight.

Read this link about used iOS devices --> https://discussions.apple.com/docs/DOC-250001038

First, was this bought used from a friend? Was it from a website? Was it from a street vendor? If the person doesn't know their own Apple ID and password that sends up red flags to me. Of course, they might have truly forgotten their log in information. You mentioned also that you can't get hold of the person any longer so that strengthens my red flag case. I hope you didn't pay the person for said iPad.

Then the iPad will not work and you will not be able to use it. Try to return it for a refund.