I would think the network would know the details of the account holder and could at least write to them requesting permission to change to your name.
As you bought the device from Craigslist, you must have contact details of the person you bought from. I take it you have at least an email address and possibly their new cell phone number - can't you simply ask them?
If you know the phone number that is registered to the cell phone that you now own, you could try a reverse number lookup.
I found this one http://reversephonedetective.us.com/ which apparently doesn't ask for payment up front and will give a refund if you aren't happy with the results.
I'm wondering if this person is breaking the law by selling a cell phone that you can not actually use without details which they failed to provide.
It may be worth contacting the police if the above doesn't give the information you need.
It certainly seems illegal for them to have sold a phone on an active plan, so that is something you could look into.
Good luck with getting to the bottom of this, and enjoy your new phone!