Simple iPhone help - I'm old?

I currently have an iPhone 4 but the case is broken and so is the speaker and all the sound. I want to buy a new iPhone 4 off of Ebay because of all the trouble my current one has. If I do that can I just switch over the SIM Card and use that one instead? If not, what do I have to do? I want a NEW iPhone 4. I do NOT want to put in the time to fix my current one.

As long as the phone is unlocked to all providers put your SIM card in and communicate!

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