Resetting a bought used iPad 2
It depends on if he signed out of his iCloud account and turned off Find my iPad first. iOS has a built-in feature so that a thief cannot wipe a device and login with his own credentials. If he did log out, then you will be fine. If he didn't, he should go to iCloud.com and reset his password if he does not know his credentials.  From there, he can turn off Find my iPad remotely so you can login. If you cannot set up the iPad, and he does not perform these steps, you will not be able to use the iPad.