I sold my wife's 16GB 4 on eBay Instant Sale. I locked in the price before pre-orders for the 5 opened, and got $200 for it (within a week, their asking price had dropped to $135). Nice because on a new iPhone release, they give you two weeks to mail the device. Got my money last week (I sent it in the week after receiving my 5). Took a little while for them to go through the backlog, but no issues. I used them last year too, and I'll probably use them when we sell the 4S next year to pay for the 5S.
Much better than dealing with potential scammers on eBay and Craigslist, which seem to come out of the woodwork on new iPhone releases.