This is partially true. If you put a DX lens on an FX sensor, it will work, but you get mad vignetting. If you've upgraded to an FX body, it kind of makes all your DX lenses superfluous.Originally posted by lewdvig
Or, someone might be moving up to FX and have one to sell (although I think Nikon FX bodies are smart enough to use DX lenses without chopping the corners.