buying an att iphone 5 from somewhere besides att store

If you're in the U.S., you can buy your phone linked to your AT&T account from the online Apple Store or a walk-in Apple Store. In either case, you will be asked to provide your AT&T cellular phone number, and that will be used to automatically link your iPhone to your AT&T account both to determine your iPhone price and to set up once you receive the unit.

Hope this helps. I can tell you I've done this many times (with each new iPhone I've gotten).

Doesn't matter where you purchase, Apple or an AT&T store, both will transfer your account info to your new phone. To transfer your data, follow the instructions here:

http://support.apple.com/kb/HT2109

Personally, I prefer an Apple store.

Since your contract is up on your iPhone 4, you should call AT&T and ask them to unlock it. An unlocked phone is worth quite a bit more than a locked one if you plan to sell it. AT&T will first verify that the contract is up, then you will get an email with instructions for unlocking through iTunes.

FYI, if you don't have plans for your old iPhone 4, Radio Shack will give you credit (my wife got $130 for a 16GB iPhone 4) to use toward your new iPhone 5.

I saw a 16 GB iPhone 4, unlocked, go on eBay for $270.

Yes. That's exactly what she did when she traded hers in. You could get more on eBay, sure, but this way there's no messing around with selling and shipping and no need to pony up the entire amount for a ne phone before you're paid for the old one.

Glad it worked out well for you!