Can I activate a iPhone 5 that wasn't bought at AT&T?

Ok I have a iPhone 4 with AT&T and was about to buy a iPhone 5 off someone who is with AT&T also. If I was to buy it what would I need to get it activated? Note: I know iPhone 4 SIM cards are different from the mini SIM cards in iPhone 5 sim slots. Would I need to take the iPhone 5 to AT&T for them to give me a new SIM card for it & will it cost me? Or will it cost me anything to get this iPhone 5 working on AT&T. It is a AT&T iPhone 5 also. Or would they not activate it since I didn't buy it from Apple Retail Store or them.

If you have a phone and its AT&T they will give you a new sim and not charge. It should work. All you have to say is you need a new sim for your iPhone 5. They'll get your info make a SIM card then your set. Should work automatically from there.

There's no way to get the phone activated without AT&T knowing.AT&T doesn't care if you bought it from Apple or someone else, but they will care if it is reported stolen, then they will refuse to give it back to you.So, I hope it's not stolen. You may need the name, zip code and perhaps other identifying information of the seller to activate. Your best bet is to call AT&T and ask them what it will take, give them the MEID or other numbers they ask for and let them answer your questions.

Also, if the iPhone was never activated at all by the original purchaser, you will have a problem that won't be fixed by AT&T or Apple, or at all.

Most carriers charge an activation fee for a phone, especially one they or Apple did not sell to you.