Legal Question in Intellectual Property in California

I really don't know what the result would be in court, especially since this would probably be a small-claims case and small-claims judges are not necessarily as knowledgable as those presiding in superior court. It seems to me that both of you were negligent in some degree, the seller for not terminating the service, or transferring it to a new phone, and you for not investigating whether the phone was "live." Also, whomever bought it from you must bear some of the responsibility for "freeloading."

One thing you might do is ask this same question, perhaps anonymously, of the managers at half a dozen cellphone stores in your area, and see which party they think is responsible and why. This might give you an indication of how a jury (or judge) might react.

Tell the seller to pound sand. She was responsible for canceling her service when she sold the phone. If you are sued in small claims court, you get to explain those facts to the judge. However, if you are sued, you should 'counter sue' the buyer and get him into this action. He is the one running up her bill.