The iPhone 3GS was originally released in 2009 and, notably, will not run the new iOS 7 when it's released this fall. For comparison, AT&T sells the refurbished iPhone 4 for $325, so the 3GS would presumably be offered for well under $300.
The tipster claims the phone will not be displayed or promoted publicly, but sales reps will be able to offer them to certain price-sensitive customers. Not all AT&T stores will have them but it is claimed that AT&T's larger and more metropolitan locations should have decent amounts of stock.
Apple discontinued the iPhone 3GS in September of last year when the iPhone 5 was introduced. The lack of support for Apple's latest iOS operating system is a concern, but the extremely price-conscious customers that would consider the 3GS could overlook that issue.