I've no idea about the value of Nikon kit, but he's a professional photographer, he takes care of his kit and if that's what he wants for it, then that's up to him.
Ok so the saving on the 50mm f1.4 isn't massive (though now reduced to £260), but the 24-70 is now £1035 and still a £164 isn't to be sniffed at.
The question is always whether the buyer thinks it's worth it.
Same as with secondhand cars, it's only worth what someone is willing to spend on it.
Looks like someone already snapped up the D700.