All this charting is interesting, but I wonder if there are factors that aren't being taken into consideration.
Since Canon USA has already dropped their price for the refurbished 5D II to $1,759, I have a hard time seeing how the ceiling for a used (no warranty) sale between two private parties can go above that. I would think that realistically, people ought to figure on at least a $100 discount from the refurbished price.
But, then again, I am always amazed at the prices people pay on eBay for used equipment. There are apparently a lot of people out there who are poorly informed consumers who don't bother to, or don't know how to, compare prices. Or maybe, it's just a different perspective, but I always feel that the value of a warranty (whether new or refurbished) should be factored in when comparing a sale from a known vendor to a private-party sale.
Given the current market prices, if I were in the market for a 5D II is would not go above $1,600 for a used model. But that's just me.