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I had a similar problem. I asked the previous owner to go into his iCloud account (https://www.icloud.com/#find), select the sold device and click on remove. After that, I did a restore and set it up on my account. To restore do:
1) - turn power off, and DISCONNECT ipad from cable, but maintain connected to computer.
2) - Maintain home button pressed and connect the cable to the ipad.
Itunes will prompt you to do a restore.
Hope this helps... good luck!
Really, a seller from Ebay. Unless he plans to ship it back to do so (I'm sure Apple Support does not recommend giving out your Apple ID and password,) so that would be the only other method available (much simplier than placing the device in recovery mode.) Just a thought.
Place iPod in DFU mode then connect to itunes. By doing that, itunes will recognize it as a device in recovery mode and allow you to do a complete wipe and reinstall of the iOS. You will have to wipe the device and update it to the current firmware to bypass the previous owners password. To place it in DFU mode, turn it off, plug it into the computer, hold down the power button as soon as it is plugged in, when the apple logo comes up then press the home button (while still holding the power button,) once the apple logo goes away release the power button and then keep holding the home button until you see that itunes recognizes a device in recovery mode, then select restore. You have to be connected to the internet and if the serial number is blogged by Appl then this will not work. If it's too hard to get it in DFU mode (better instructions are online,) you can place it into recovery mode by turning it off, hold the home button while you plug it into the computer and keep holding the home button until the connect to itunes animation appears on the iPod. If you choose that method, the first retore may fail and you will have to do it 2 or 3 times but both methods will work to bypass the previous owner's password.
Based on this Apple support page, it sounds like you'll need the previous owner to log in (or give you their log-in info) so he or she can remove the device from their account. This appears to be the case even if the device's contents have been erased.