I bought an iPhone on eBay and it is not released from their account? Sprint?

I bought an iPhone on eBay but I can't swap ESNs because it is still on their account, how can it get released or how can I use it on my account?

I don't have their information to contact them anymore.

The original owner would have to pay the balance in full and finish the 2 year contract.
until them you have a bad esn phone and it can only be used as a ipod touch.