Confusion! when it says "iPhone 4S AT&T", does it mean that the 4S was locked to AT&T GSM & unlocked CDMA? I saw an add in Amazon
http://www.amazon.com/gp/offer-listing/B005SSB0YO/ref=dp_olp_new?ie=UTF8&amp;condition=new

It says "Apple iPhone 4S 16GB (White) - AT&amp;T", so it means that that iPhone only works for AT&amp;T. SInce AT&amp;T uses GSM so that phone can not be used for any other GSM carriers right?

Ok, but the iPhone 4s also supports CDMA &amp; CDMA do not need any sim card, so will the above phone work with any CDMA carrier? 

It seems that even 4s supports both but if the phone was sold as locked phone then the CDMA part was disable in 4s, is it true?

And via versa, ie if the 4s was locked to Verizon then the GSM part of 4s was disable &amp; that phone can not be used for GSM net work, but it can be used for GSM in oversea where Verizon has no competition. Is That true as well?
(http://www.cnet.com/au/news/why-you-cant-take-your-unlocked-iphone-4s-to-another-u-s-carrier/)
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.