Does Sprint use SIM card for postpaid?

Well, u bought an iphone 5 on ebay. I asked the seller if it's clean esn, but he only told me that I'm just gonna need a new SIM card and that the store has it. I'm on a post paid plan. I'm just wondering since the seller said I'm gonna need a new SIM card if it's probably prepaid and some people online said that you cannot do prepaid to postpaid. Can I still activate the phone as postpaid?

Yes, they use SIM cards for post paid users.

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