Can a Samsung BlackJack 2 cell phone, without a SIM card (unlocked) be used as an AT&T pay-as-you-go phone?

I really wanna buy the Blackjack 2. The seller is willing to unlock it for free. I need to know if I get them to unlock it, if I can use it on the AT&T pay-as-you-go plan. Wouldn't I use top-up cards? Thats what I want! If I do this, do I still need a SIM card? Could someone please explain this to me, help me out. Thanks! :D