eBay is your friend. You'll get a pretty good price on the old one, too...the demand is high from unlockers.
Before you sell the old one, be sure and do the full wipe on it (takes 1 hr per 8GB) to make sure your data is securely erased.
From the iPhone documentation:
Erase all content and settings: Connect iPhone to your computer or a power adapter. Choose General > Reset and tap Erase All Content and Settings.
This feature resets all settings to their defaults and removes all your information and media on your iPhone by overwriting the data stored on iPhone.
You cant use iPhone while data is being overwritten. This can take one to two hours or more, depending on the storage capacity of your iPhone.