Can I buy a used iphone and transfer it to prepaid account?

I was wondering if I could buy an already used Iphone 4s or Iphone 5 from like EBAY or where ever and transfer it to a prepaid account.
I really don't want to pay $500 for a phone and I don't want to get stuck in a 2 year contract.

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Yes you can but just make sure that the iPhone you are buying is from the same phone company that the one you have.