Best you can do is try to track down the former owner. Generally there's a message that comes up when you plug it into iTunes, from the previous owner, with contact info, etc. If it's there, try contacting them; if not, you've got no choice but to return it; it's a fancy brick without the proper login info, unless the owner removes it from their account.
Naturally this is entirely shady and illegal, but if there's contact information, you can try and extrapolate the available info to figure out their security questions and their iTunes email, then reset the password, log in, and remove the iPad from their iCloud. I managed to do so with nothing but the previous owner's cell number on an iPhone 5 I bought from eBay, but the only reason it worked was a combination of an unusual name, easy security questions, and being lax about Facebook privacy settings.