activating iphone from ebay or craigslist

There isn't a way to insert a SIM card into the iPhone, but there is one in the iPhone already. All you need to do is dock the iPhone to your computer with iTunes installed and iTunes will recognize it and guide you through the process to activate the phone through AT&T as a new or existing customer. I would imagine that the person who sold it to you will have already activated a different cell phone or deactivated their iPhone in some way so there aren't any hangups, but I'm sure an AT&T rep can help you with it if necessary.

Sorry, but that's wrong. You can't use the SIM in the phone if it has already been activated with that SIM. You get a new SIM card from an AT&T store (they may or may not charge you for it; usually they won't, but they are only $25 or so if they do). You remove the SIM card that is in the phone by inserting a paper clip in the hole on top of the phone next to the earphone jack. You put the new SIM card in the phone, then activate it through iTunes.

Actually, there is a way to insert a sim card. On the top of the phone is the piece that comes out which houses the sim card. There is a small hole where you'd stick a pin or paperclip to open it. Anyway, you wouldn't have to buy a cheap phone to use its sim card for the iPhone you get from eBay or Craigslist, all you'd have to do is go to an AT & T store and ask for a replacement sim card. They don't generally charge for them at least that has been my experience plus there have been posts here from individuals who have also stated they've not been charged for a new sim card. Then you would just hook the phone up to your computer/iTunes and it will take you through the activation process.

Hope this helps.

iPhoNettie

You will have to call AT&T for an answer to that question. I know the 3G phone will only be available on contract, and not on a Gophone plan. I don't know about the Classic iPhone.