From my experience, you can't. I have had that trouble with Sprint before. CDMA phones which don't use sim cards are locked by the MEID. Until that account is cleared by the previous owner you can not re-activate that phone.
Basically what happened is that person still owes them money for their bills or contract termination fees and refuses to pay. They lock the account until it is paid by them.
Do you still have the information on the person? You could contact the local law enforcement and file a report with their cyber crimes division and they can subpoena Craigslist for the contact information on the seller.
My recommendation for the future is get the MEID from the seller before purchase and call the provider to make sure it is ok to activate.
Sorry I could not be more helpful. Good luck.