Apple implemented the Activation Lock feature with iOS 7 to reduce the problem of stolen iPhones, iPods and iPads. This feature essentially bricks any lost or stolen iOS device until the rightful owner enters his iCloud password and removes the device from his account.
I suggest that you contact the previous owner and ask them to follow the steps below to remove that iPod from their account. Otherwise, you will not be able to use the iPod. If the seller does don't have this information, I would demand a refund and possibly contact the police because you may well be in possession of stolen goods.
If you can reach the owner ask them to remove it from their account:
1. Sign in to iCloud with their Apple ID.
2. Go to Find My iPhone.
3. Select All Devices to open a list of devices linked to their account, then select the device to be removed.
4. Click Erase iPhone. When prompted, don't enter a phone number or message. Select Next until the device is erased.
Click Remove from Account.
BTW, there is a chance the owner could use the "find my iPod" feature to see where it is located and ask the police to come by your house (or wherever the iPod is) if in fact it is stolen.