Hello pchu1701,
It sounds like the seller didn't take these recommended steps when selling their iPad: What to do before you sell, give away, or trade in your iPhone or iPad
We would recommend contacting the seller once more to have them take the steps under section, 'If you no longer have your iPhone or iPad' below:
If you no longer have your iPhone or iPad
If the steps above weren't completed and you no longer have your device, use these steps:
1. Ask the new owner to erase all content and settings using the above steps.
2. If you're using iCloud and Find My [device] on the device, sign in to iCloud.com/find or the Find My app on another device, select the device, and click Erase. After the device has been erased, click Remove from Account.
3. If you can't follow either of the above steps, change your Apple ID password. This won't remove personal information that's stored on your old device, but it prevents the new owner from deleting your information from iCloud.
4. If you're switching to a non-Apple phone, deregister iMessage.
5. If you're using Apple Pay, you can remove your credit or debit cards at iCloud.com. Choose Account Settings to see which devices are using Apple Pay, then click the device. Next to Apple Pay, click Remove.
6. Remove your old device from your list of trusted devices.
7. If you purchased an AppleCare plan for your device, cancel your AppleCare plan.
If they refuse or can't reset their Apple ID password, then it would be best to discuss your options for returning the product with the seller.
Take care.