Buying iPhone 4 off of ebay

Hi
i currently have an old pantech ease phone with att.
i wanted to upgrade to an iphone, but the att store said my contract isn't up until october.
there's an iphone 4 for $250 i'm interested in buying from eba and had a few questions

1. Can i just cut down my 3g sim card and put it in the iphone 4?
2. Do i have to have a data plan?
3. If i put my cut down sim card in the iphone, will data charges automatically be charged to me?
4. My phone and the iphone are both with att. Will i have any issues with it recognising the sim?
thanks

Added (1). Oh also. If i get the iphone and they charge me for data… Will my contract still be up in ovtober or will it add on 2 more yearS?

Why not just do a 2 year contract with iphone 4 and get it free? Thats alot better then buying used.

Don't buy phones from ebay as they could be fake!

$250 is too much for a 2 year old iphone.
it will no longer be supported when 6.2 is released in april.
you can find a 4 on craigslist for $150-$200 tops.

1… Yes while you can cut down your SIM, at&t will give or sell you a new SIM cheap

2… AT&T will just start charging you a data plan after you start using data

3… YES, you will be charged MORE money

4… If your iPhone is an AT&T phone then you should be fine

5… Your contract will no change, not be renewed if you buy a phone from AT&T or ebay.

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