Confusion! when it says "iPhone 4S AT&T", does it mean that the 4S was locked to AT&T GSM & unlocked CDMA?
The only place to buy an unlocked iPhone is direct from Apple. Buying one from anywhere is a risky proposition. It may either not work as advertised, or worse yet, be a previously stolen phone.

Even though you are correct that the iPhone 4S has a chip that works on both GSM and CDMA networks, per Apple, the unlocked phones will only work on GSM networks.