If i buy an iPhone 4s can i put my old sim card in the phone?

I want to buy a used iPhone 4s because my previous one broke. Can i just place my old 4s sim card int o the one i purchased or do i have to activate the phone all over again?

I believe you CAN transfer your SIM card. But note all new iPhones come with a brand new SIM card.

Both phones carrier same means, No problem. You can swap your sim card from your old iphone 4s to used iphone 4s.Locked by other than your current carrier means you will need to unlock your phone. For unlocking visit http://www.prounlocking.com/apple-iphone-4s/rs5wp9/ Here you can unlock your iphone by Remote unlocking method without jailbreaking.