Either here or swappa. Swappa is nice because if you know what model you want you can compare prices pretty easily. Just be careful because a lot of the phones are rooted. Most sellers will return back to stock if you want.
On this forum you just need to monitor the classifieds. If you see something that interest you then check the price sharing swappa and see if it compares.
With both sites don't be afraid to haggle a bit, just don't insult the buyer with a really low offer.
Past Devices: Google Nexus One (sold), Samsung Galaxy S2 (sold), B&N Nook (returned), Samsung Galaxy Tab 7 (sold), Samsung Galaxy Tab 10.1 (On the Shelf)
Current Devices: Samsung Galaxy S3 (AOKP), Nexus 7 (Stock), Chromecast