I found a little used Nikon 200-400mm f/4 VR on my local craigslist. A local doctor had bought it and used it twice. I even bought a used D300s to use playing with it, and a gimbal head, and was still below $3300. Considering how over priced Nikon lenses usually are, I'll have no problem selling for a lot more if I ever do it.
That is the only Nikon lens I wish Canon had in its line-up (next to /other than the 200-400 1.4x). A former colleague of mine has it and it is absofabolutelyfantastic
5D3, 5D2, Sony α6000, G16 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: SY12 f/2, Ʃ19 & 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses