You will need to contact the eBay seller so they can deactivate it from their account. If the phone is theirs, they can deactivate it within 5-10 minutes. If the phone is being resold then they will need to issue you a replacement. The good news with eBay/Paypal is that it is very buyer friendly so you can always leave poor feedback or start a dispute in order to resolve an issue. Most sellers want to avoid that scenario so they will do their best to fix the issue.
If you can take a minute and use the rating system to rate our session as 3 stars or higher and press Submit so I am credited with helping you.
You can contact AT&T and determine if the phone is still active. I would suggest that you contact them to see if it is still active and if it is then reach back out to the seller and let them know that you want to return the phone because it is still active on a network. If the seller cares anything about their feedback score (and all full-time sellers do) then they will make the issue right.
Another question -- Who is telling you that it is active on AT&T network?
Sure-- You can call 1-***-***-**** to reach their support team. When you speak with an agent, tell them that you purchased the phone online and you want to see if it is active on a plan. Let them know that you think you were sold a phone that was still active so they can double check it.
No, there isn't.
I would suggest that you contact the seller again and let them know that it is showing up as still active through your unlocking agent.
I have not heard back from you in a while -- If the issue is resolved, please rate our session as 3 stars or more and press Submit so I am credited with helping you otherwise please reply back so we can continue with the issue.