Swappa: Swappa is an Android-only site for buying and selling, and they go out of their way to state that they are merely a facilitator of person-to-person sales. As a result, you can get more for your device if you know what is special about it: lots of accessories, extra batteries, modified to run a newer version of Android, and so on. The front page shows a Verizon Galaxy Nexus starting at $280. You'll have to do a bit more to prove your veracity as a seller (including taking a picture of your device with your seller code written out on paper), but the right device can fetch a nicer price.
RadioShack: Seriously. Did you know you can trade your phone in at RadioShack? Online, or, if you're leaving town and need to drop off your phone right now, they'll take your iPhone, or Android, or other device, and give you money for it. $300 for the iPhone 4S isn't too bad, in any case.