I am thinking about getting an iPhone...?

I am using a normal Samsung 3g phone (SGH-A747, AT&T) as of right now, but I am thinking about buying an iPhone 3g because I'm starting to get tired of my Samsung phone. A few questions: a. Will I have to sign a contract even if I don't buy it from Apple or the AT&T store? Like buying one on Ebay or Amazon,... show more I am using a normal Samsung 3g phone (SGH-A747, AT&T) as of right now, but I am thinking about buying an iPhone 3g because I'm starting to get tired of my Samsung phone. A few questions: a. Will I have to sign a contract even if I don't buy it from Apple or the AT&T store? Like buying one on Ebay or Amazon, or even Craigslist? b. If I end up getting an iPhone 3G, (not at the Apple or AT&T store), will AT&T start charging me more per month? The internet is disabled on my normal plan as of right now anyway so I don't have to worry about internet fees. c. Will my 3G sim card work with the iPhone or do I have to get a specific one? I heard all you have to do is switch them around but I don't know if any complications would arise from doing simply this.

Thank you for your time.
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