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 | EF: SY14/2.8, V20/3.5, 28/2.8 IS, Ʃ35/1.4, 50/1.8, Ʃ50/1.4 EX, 100/2.8L IS Macro, 16-35/4L IS, 24-105/4L IS, 70-200/2.8L IS II, 1.4x II, 2.0x III, 70-300L IS, T28-300 Di VC PZD, Ʃ150-600 DG OS HSM S | E: SY12/2, 35/1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY300/6.3 ED UMC CS