For general used pricing guidance, I recommend checking three places:
1. Canon USA Refurbish --- This is the max amount I'd pay used considering you have to pay local sales tax and shipping, but, in return get a one year Canon warranty. Granted the current refurb price for the 70-200/4L is $568, which is too close to the new price...
2. www.keh.com
--- You don't get the box or warranty card, but, you get prices vs. lens condition (assuming that particular lens is currently available at that quality level).
3. Craigslist prices. If nothing else, this tells you how much people think they can sell lens for when they come with the box and warranty card, and sometimes with receipt (especially if the lens still is under its original warranty).