If you guys are still selling your phones, I would recommend putting an ad up on your local craigslist. Before I dove into the root world and unlocked my SG4G to its full potential, I posted an ad up to sell my phone for $200 and got all kinds of offers. This was about two months ago.
Sad to see many people moving away from this phone. Although I do believe it is becoming outdated, I still think its a great phone nonetheless. I'm personally holding onto it because I will be spending a year abroad and have already unlocked it for international use.
Best of luck.