Given the fantastic Christmas sales on 5D IV, when they were going new with Canon warranty for under $2,000, it's no surprise the used price has fallen.
On the other hand, the Adorama offer on the 1Dx II is about $1,000 less than what they have been selling for on eBay. It's like trading in a used car vs. selling it. What price are you willing to pay to a seller not to have the hassle of selling it yourself? To me, $1,000 seems a bit steep, especially since I doubt the value of the Mark II is going to drop that much with the release of Mark III.