I'd recommend contacting all the carriers that this phone is compatible with, and tell them that the phone has been stolen. Give them the IMEI number, serial number and whatever else.

If you can get law enforcement interested in the case (highly unlikely, but who knows), they should be able to issue a search warrant, and get the location of the phone from the carrier. If they can track it down to a residential address, then they may be able to find the actual person.

It would be helpful if the other sellers also had remotely trackable items. Regardless, you'll want to be coordinating with them on the item descriptions.