I'm going to have to agree with this. Twice in the last three months I've had issues with third party sellers on
Amazon. While Amazon will ultimately back you in the end you pretty much have to jump through hoops w/the seller first before they will intervene. Unless it says FULFILLED by Amazon I am staying away from third parties from now on. Too much hassle.
Myself, I wouldn't be comfortable buying a phone from someone other than a vetted source like Swappa, Gazelle, Game Stop,
Best Buy, Apple etc. Somewhere where I have recourse right away if something should go afud. That said, many have great success with eBay. Things to keep in mind:
Ask for the IMEI #. If they won't give it to you, move on. If they're willing to give it to you don't just go on that actually check it out. This will tell you if the phone is free and clear.
http://www.imeipro.info/ You can also check to see it works with your carrier here.
Verify that they have removed the phone from their iCloud account. This is usually the biggest problem and an inexperienced third party often forgets to do this.
Last, new phones should be announced around September so the price of the 7 should go down. Something to consider.
Addressing a phone through AT&T. If the phone is paid for it should automatically be unlocked. If someone is selling the phone theoretically it *should* be paid for therefore unlocked but there are those out there who will sell phones still being financed. Read reviews, make sure they're experienced with selling phones.