Guys - you all took my comment the wrong way. What I meant was that the OP made an error by not unlocking the phone BEFORE selling it and getting his extra $100.
I don't think the buyer is acting like a jerk either. He's just asking if the seller would unlock the phone. There is no cost involved, so why not just do it? I didn't see anything about threats of negative feedback from the buyer.
The transaction is complete. I don't why the OP would want money from the buyer to have him unlock the phone, when it doesn't cost the OP anything.
That's not an error. That's choice.
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I, along with many others have had the opposite experience.
If those phones were still under obligation att won't unlock. The last time I did this was with a 4 from Craigslist. The seller had finished his contract with the phone, this it was eligible for unlocking.