Check it out:
In addition none of those market places support the Android community the way Swappa does.
Throw in some other small features that really make the service shine. Like quick Facebook and OpenID login and profile creation, dead simple instructions for buying and selling and the knowledge that you are getting a good device at a great price. There really is no better way to buy or sell an Android based device, phone or tablet.
If you haven't already, check out Teknami and the related FatHat Games.