I put my 64GB At&t 4S on craigslist for $450 and had a buyer withing 3 hours. We met at a Starbucks. Guy was really cool too. No fees or any of that crap with shipping. I drove 10 min from my house.
Still looking to sell mine. Have a 32gb white and black in mint condition. Put it up on craigslist but will probably end up going up on XDA. I've always had luck there.
try swappa.com ; how much are they going for on XDA?
I'm curious what a black 64 gig factory unlocked 4S would go for.