Given what you have said, you have no options other than to recycle the iPad.
In detail:
If you see a Activation Locked (iCloud locked) message read this Activation Lock removal article --> How to remove Activation Lock - Apple Support To remove the lock you will need either:
OR
-The original sales receipt from Apple itself or an Apple approved vendor. Other ways you may think might be “proof” of ownership or purchase, such as having the box, or a receipt from a third party vendor such as Ebay, will likely not be acceptable by Apple. Apple may have no way of verifying the authenticity of other methods. If you have an acceptable receipt, ask Apple to unlock the device —> https://al-support.apple.com/#/additional-support
Recycle the device if you cannot unlock it; it is useless to you. From the very bottom of Apple Trade In - Apple :
"Does Apple offer recycling?
Yes. Apple Trade In lets you recycle any Apple device (including devices from Apple-owned brands) at any Apple Store and on Apple for free.