or do nothing of the sort, take your money and buy some beers or whatever. you did nothing wrong, or dishonest, and if the buyer tries to tarnish your ebay feedback, go after him. it's not worth the effort to correct someone's buyers remorse. he knew what he was buying and bought it. you sold it. you're done.
+1
Inform inform him very plainly that he received the phone as it was advertised and that if he wants the phone unlocked he needs to PayPal you $75 for that privilege.
These are two separate transactions; the first was for a locked AT&T phone, the second is an unlock process. He is not asking you to do something unreasonable, however he is not offering to pay you for it and perhaps he does not realize that an unlocked phone is worth more than he paid. Give him that option and put the ball back in his court.
BJ