Sprint(and companies using the Sprint network) doesn't use SIM cards, the phones need to be activated by the company. You'll have to get the device, then call(or use their website if available) and input a code that's on the phone, usually under the battery.

So there is a risk with buying used phones under this system as if the phone is reported stolen it's deactivated and can not be activated again. So you run the risk of buying a phone that is practically useless and won't know it until you get it and try to activate. I'm pretty sure there are ways to check if a phone is locked or not, but I don't remember so you might have to do some research.