The previous owner doesn’t know what email they used the the iCloud and now there’s an activation lock on the ipad

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.

Sorry - but you’re short of options.

Hello,

The iPad is locked with Activation Lock. There is no way around unlocking Activation Lock without inserting the device’s associated Apple ID. Most likely if they "forgot" their Apple ID then there is a good chance that you purchased a stolen iPad. There is nothing you, or Apple, can do to unlock this iPad. You have 2 options, return the iPad for a refund or recycle the iPad.

Activation Lock

Apple Trade In - Apple

Then get your money back. The previous owner didn't sell you an iPad which was properly prepared for you the next user. If they are not able to remove the iPad from their account, as evidenced by them not knowing their Apple ID, you cannot use the device. Period. Apple won't help you. We can't help you.

Return the iPad for full refund and if they can ever get the iPad ready for a new user, you can buy it again.

See this link about buying used iOS devices --> The All Too Common SAD Reality of Buying … - Apple Community