NO!! if your gonna use a at&t prepaid sim card in a at&t unlocked phone, i wouldn't do it! people on ebay charge way more for an unlocked phone then they would a locked phone so if i were you, i would buy a locked one because your getting the same phone but a better price
An unlocked phone is so you could use a at&t phone on tmobile or other way around, not so you could use it with prepaid, you could do that anyway