It's not easy to locate on their site, but Amazon has some of the highest trade in values.
Catch is, you need original everything, and in pristine condition.
If you do, and the price is good enough , it might be something to consider.
Personally, a 4S on AT&T is what I'm shopping for.
Should be a flood of them by Xmas, as owners hit 2 years, and trade up to 5S (or others).
However, if I'd had a 4S this last 18 months, I might be looking at an S3 or S4, too. :up