I've only traded a couple games (seriously people... gift receipts!!!), and even with the price of shipping materials, Amazon gave the best payout. I do a frightening amount of buying from them, so it's all good.
I only traded in one game this year... Fallout 3 (Preemptive strike for the GotY edition) and Amazon offered me far, and away, more than eBay or Gamestop was offering.