If your not in a rush, your better off selling them on half.com or amazon.com
I typically price them to get rid of them and give free priority shipping for media mail. It gives me free confirmation, and if anything comes up I have proof of deliver. Plus customers get it quicker, so less complaints.
At the end of the day I probably get over double what I would have gotten for trading them in. Not to mention it just gets desposted into your bank account