Activate iPhone 4 from ebay?

I currently have an iPhone 3 through ATT. I just won an auction on ebay for an iPhone 4. I'm pretty excited. However, I'm wondering what I will need to do once I get it to activate it? Is there something that I need from the seller? Is it as simple as just moving SIM cards? I think I've read somewhere that I need to sync it with iTunes… Is that difficult? My 3 I bought through ATT and they activated it for me. I'm afraid they won't help me with this one since I didn't buy it through them. ANY help would be appreciated. Thanks!

Added (1). How do I know if it needs a microSIM card? I hope ATT helps. Last time, I brought them my Blackberry and my new iPhone and they did everything for me. I'd love to do that again. Are SIM cards expensive? Like, if I had to buy one, about how much do they cost?

First, you must make sure that the iphone4 doesn't use the micro sim card. If it does, you can go to AT&T and ask for it.
To activate it, all you have to do is switch the sim card.
To sync with iTune, you just need iTune in your computer, and connect the phone to iTune with the charging cable.
AT&T will help you out they don't want to loose you as a customer.