If the seller didn't take their apple ID off and that iPad is running iOS7 then there's nothing you can do. iOS7 came with the 'activation lock' meaning that it requires the original owner to unlock it with their apple ID.
If your device is pre iOS7 then you just need to go into the settings, erase all content and settings and wipe it. And honestly, the seller should have done that. If not your reseller the original owner should have wiped their info off the device and your reseller should have made sure of that.
If that iPad is running iOS7 and it's still got the original owner's apple ID on it, take it back and get your money back because you can't activate it without having that info (and it's possible that your reseller is selling something that they shouldn't be)