If you took good care of your iPhone 4S, you can get $500 from Amazon.com's Trade-In service. Gazelle is a pretty big rip-off, almost as bad as a pawn shop. Personally, I'd still use Craigslist and get $600 or more for that iPhone 4S, but if you get no buyers within the week, I'd sell it to Amazon if I were you.
Even after the 4S was out ppl were still dropping 400-500 on the 4 lol
Me I'm planning on selling it on either eBay or Craigslist for 400 and taking that to cop the new iPhone 5