You're contradicting yourself. You bought an unlocked phone on ebay, but it's locked to AT&T...
In other words, you bought a hacked/jailbroken phone on e-bay. It may not be possible for you to process a legitimate unlock, even if AT&T agreed to do so, which they will not if you are not a current/former customer or can't provide proof of purchase for a no commitment phone from an authorized source.
We can't help you. Discussing jailbroken phones is forbidden by the terms of service.