Nokia Lumia 1520 Unlock denied

Hi,

I bought an AT&T Lumia 1520 which the seller on eBay claimed was "New". Of course, I don't fully believe that, but I assumed it must be at least refurbished.

The seller also said in his description and title that this phone was "AT&T Unlocked".

When the package arrived, the box looked non-standard and said just "Lumia 1520" with no Nokia or AT&T brand trademarks on the box, which was a bit odd. However, the AT&T screen protector (about no texting and driving) was still on the front glass and the phone looked unused, with no scratches and no other signs of use.

I put my H2O wireless SIM card, and it worked right away, being an AT&T MVNO. A day later, I decided to verify that the phone is indeed SIM unlocked.

I requested that the phone be unlocked by going here 

https://www.att.com/deviceunlock/request/#/nonattunlock. 

It showed me the message -

"Unfortunately, your unlock request was denied because:Your device is currently active on another AT&T customer’s account."

What does this mean? Is this phone stolen? Or did the phone get associated with my H2O wireless account? 

I removed the H2O wireless SIM from the Lumia and put my H20 wireless SIM into a different phone and used it for a day. Same results. 

I don't have an active T-Mobile SIM, but I tried with an old T-Mobile SIM that's long unsubscribed. The Lumia shows that there's "No service" and gives absolutely no response to ##782# to unlock. 

How do I figure this out. I don't want to be using a stolen phone. The seller is not responsive. 

Would AT&T customer support help me out to verify that this phone has not been reported lost/stolen and also that there isn't any AT&T customer who has the same IMEI somehow? What option do I choose to reach customer support, given that I'm not an AT&T customer?