Good advice here, especially about really considering if you want to do ebay.
It used to be easy to sell iPhones but now, there are a lot of people who are out to take advantage of others. Also, eBay/paypal are complete *******s to sellers. If there is a dispute they will almost certainly side with the buyer. Buy shipping insurance to protect yourself.
One more thing...bidders with 0 feedback ratings....put in your listing that you won't sell to them and that you will cancel any bids from buyers with 0 feedback. I have never had a good experience with them and I am about to re-list my 4S because someone with no feedback (once again) has apparently flaked on me.
You might want to consider not selling internationally also. You will have to put this in your listing as well becuase invariably, some dude from ubekakazekiopikastan will win your auction and ask you to ship it to some impossible place to reach, and when you tell him it costs $200 to get the phone to him he will likely disappear, leaving you in limbo for days while you wait for eBay to process your no pay claim.
Just my $0.02. Good luck.