Do not sell your used iPhone 4/4s for the next month or so. Everyone is offloading them at this time and you will not get much. In a month or 2 the used market will dry up a little and you get get $50 to 75$ more at this time.
If you don't need the money to upgrade to the iPhone 5 just keep your old one for a little longer.
If you don't need the money to upgrade to the iPhone 5 just keep your old one for a little longer.