Are you certain that the iPad is not Activation locked by your own AppleID? Simply using Erase All Content & Settings will not release the Activation Lock. The Activation Lock must be released before erasing the iPad:
Settings > [Your Name / AppleID] > Find My > Find My > Find My iPad > Find My iPad - set to OFF
After Find My iPad has been disabled, you can then erase the iPad.
Assuming that the iPad is still associated with your own AppleID - and you have not since manually deleted the iPad from your AppleID account - you may be able to resolve the problem:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
Follow the section entitled "If you no longer have your iPhone or iPad".
If the iPad was using your own AppleID prior to being erased - and you manually deleted the device from your AppleID account - the only way to resolve the issue is to recover the iPad from whoever purchased the iPad from you. When the iPad is back in your possession, sign-in using your own AppleID; this will re-associate the iPad with your AppleID account - after which you can properly prepare the iPad for disposal and reuse by its new owner.
Otherwise, if the iPad disabled or Activation Locked by an AppleID that does not belong to you, unless you can independently obtain assistance from the previous owner, the iPad is (and will remain) useless.
Activation Lock
For information, here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be satisfied:
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
Turn off Activation Lock – Apple Support
As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as the original sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.
Here is what Apple suggests when purchasing a previously used device:
Check for Activation Lock before you buy a device from someone else
Before you buy an iPhone, iPad, or iPod touch, from someone other than Apple or an authorized Apple reseller, make sure that the device is erased and no longer linked to the previous owner’s account. Follow these steps to check if the device is ready to use:
- Turn on and unlock the device.
- If the passcode lock screen or the Home screen appears, the device hasn't been erased. Ask the seller to completely erase the device by going to Settings > General > Reset > Erase All Content and Settings. Don't take ownership of any used iPhone, iPad, or iPod touch until it's been erased.
- Begin the device setup process.
- If you're asked for the previous owner’s Apple ID and password, the device is still linked to their account. Hand the device back to the seller and ask them to enter their password. If the previous owner isn't present, contact them as soon as possible and ask them to use a browser to remove the device from their account. Don't take ownership of any used iPhone, iPad, or iPod touch until it has been removed from the previous owner’s account.
You will know that a device is ready for you to use when you're asked to "Set up your iPhone," “Set up your iPad," or “Set up your iPod” the first time you turn it on.
