Unfortunately, it's probably going to be reported stolen (if it was even REALLY his to begin with). When I've dealt with cell phones and craigslist, I met the user AT the providers store. They will do the esn verification right there, and it protects both buyer and seller. If he was looking to use the money to go towards a new phone, he can see the $$ is there, and get it right there at the store, and you know it's clean since it was just checked and added to your account. It's also "neutral" ground with witnesses if anything should go wrong in the transaction.
Sorry about the loss there man.