Buying a used iphone

[desura]

So a guy at work wants to upgrade to the newest iphone. He has a 4s 16gb. Looks in about good-ish condition.

How much to offer? How much will ATT offer? Should I insist on it being unlocked? If so...how difficult would it be for him to get it unlocked?

Top of my head I'm thinking $200.

 

[pm]

I will probably annoy you a lot by locking your post and pointing this out, but price checks aren't allowed by forum rule #14.
http://forums.anandtech.com/showthread.php?t=60552

I suggest hitting Ebay and looking at completed listings. (but for what it's worth, $200 for a good quality iPhone 4S is pretty low... he could do better with Amazon's trade-in program).