I don't have very good input, but I am in a talkative mood today. By Locked In, do you mean you do or don't have a deadline when you need to send it in?
We all know there WON'T be a new phone for atleast 22 Days. Do you have a backup phone? I have AT&T so I have like 4 backup phones.
Sometimes you just have to bite when opportunity knocks. A few years ago I sold my iPhone 3GS before the iPhone 4 was even announced. But I was so confident in the release date due to internet rumors. Talk about looking forward to the Keynote! I had a buyer that was willing to pay $450 for it. I took it. And used some old Sony for nearly 45 Days until FedEx came a'knockin.
Be sure you send it in with enough time to get money to buy a new one instore, or pre-order. Pre-Orders can begin as soon as September 13th if the site does not crash. I say get the money by Monday, September 10th. (I don't know the processing time on Gazelle)
Good Luck, and I can't wait to be talking about our iPhone 5's.