I've been a keen observer and participant of Craigsliat for many years and the patterns are always the same.
Immediately at launch the resale value of the previous gen drops like a rock. Instead of getting $375 you'd have to list at $250, for example.
But once hype dies down in 1 to 2 months or so the price rebounds a little to say $325 or $350.
It's simple economics, supply and demand. There's always some people looking to buy used iPhones. The number of used buyers doesn't change with launch dates because they're buying used so they're clearly not early adopters.
However the supply increases dramatically because so many people who have the latest model want to keep up and get the newer one, so they list theirs along with the usual weekly listings from random people.
When demand is fairly constant and supply shoots up dramatically, the market price drops. This does encourage some to enter the used buying market to take advantage of the price drops but these entries don't offset the spike in supply.
So if you want to rid yourself of the phone, drop the price and sell fast or trade in. If you want maximum value, hold off selling for a few weeks to a month and a half or so.
Econ 101.