Going by the pictures it doesn't look absolutely new (the paint finish on the focusing ring seems to have a bit of "patina") but does look very well-cared-for. It's certainly possible to find older lenses in almost-like-new condition with the original box and papers. When I buy equipment new I always save the boxes and papers, and I'm pretty careful with it, so I've occasionally sold things in similar condition to your 105 (although not quite as old.) So the condition of this lens is unusual but not implausible, I'd say.
In fact, I've heard it said that older Nikon equipment generally is either very rough or nicely preserved, seldom in-between. The rough items were those bought by professionals and used heavily over many years. The nicely-preserved ones were bought by fastidious amateurs (often servicemen taking advantage of Nikon's sales through PX shops) who wanted to have good equipment but used it only occasionally and stored it carefully between uses.
