Buyer already knows that it's a used set so there's nothing wrong for the seller to sell him a demo set. Like someone already pointed out, if the seller claims the item is BRAND NEW but sells you a used item, then it is clearly WRONG.

Looking at the seller's thread, it is clearly mentioned there that the condition is awesome http://forum.lowyat.net/index.php?showtopic=2329241&hl=

It could be misrepresentation by the seller because it clearly isn't in an awesome condition as there are missing screws and defects in the lens. I said "could be" because the defects were discovered a while later and it is unknown whether the seller was aware of such defects and knowingly sold it to TS.

If the defects were discovered immediately when TS received the item, then TS definitely has the right to ask for a FULL REFUND. But it was only discovered a month later.....it could also be contributed by TS himself (defects in the lens, NOT the missing screws)

So, now, still quite uncertain about this

It's not a requirement to state whether he got it from someone else or not actually. As long as the seller indicates that it's a USED unit, it should suffice

Time to go make noise at Nikon Service Centre.

If that doesn't help, make noise at their FB page.