I bought a phone from someone on craigslist that I can't use because it is on their account.

(1) Send them a certified letter, return receipt requested, demanding that they contact the telephone service provider to notify them as to the change in ownership of the phone and to release the telephone from the account. Demand that they do so within ten days, otherwise, that you will seek legal remedies as required.

(2) Contact Craigs list. They have a procedure to report fraudulent sales. This is a fraudulent sale if the seller took money for a telephone you can not use. Notify Craigs list that you are the victim of a sale of a product that is unusable due to the faulure of the seller to take appropriate action.

(3) Failing (1) and (2) above, you will then be required to file a small claim against the seller in the jurisdiction in which the seller lives. If you bring it in the closest most convenient court to you, the case will be dismissed unless it is also the court in the jurisdiction in which the seller lives.

Finally, there are laws pertaining to unfair and deceptive acts and practices in the conduct of trade. Unfortunately these laws apply generally only to commercial transactions or sales by dealers, and do not apply to private sales.