Question [Q] Sprint CDMA instead of GSM

May I know, the one I bought now is a CDMA HTC Evo 3D and the seller said it is using Sprint.. is it usable to make calls and send text messages if I fly to America?

How long will the Sprint line be usable before it finally gets blocked (cannot call out or send text messages?

And lastly, would it mean that the seller will be billed shall I manage to place calls or send text messages?