Best bet is the Nikon store somewhere in V-Mall if you're going to buy at Greenhills. I think meron rin sa SM North CyberZone.
For used lenses, I have bought and sold some stuff at TipidPC. While I can vouch for some sellers there (those who are not higher-ranked in feedbacks), yung iba, at times, are overpriced. BUT, you do get value for your money. Ang hassle lang ay yung usual stuff like hindi dumating yung kausap mo, may hidden faults pala, etc... like you would usually find in a b/s forum like that and this site right here.
I also don't suggest buying second-hand lenses/cameras at Greenhills (esp the pseudo CyberZone connecting Shoppesville and V-Mall) most of them are not "qualified" (for a lack of a better term) to sell those items. They do not know what they are talking about. A friend of mine got ripped-off buying lenses there, he had no technical questions to ask and was taken by the sales talk. Sabi ko sa kanya dapat sa Hidalgo na lang or, at least, kasama niya ako. While there was a fault in my friend's lack of knowledge, I am sure that this won't happen in Hidalgo.
May second hand din pero risky.
Ako sa robinsons forum ako bumibili. Hanapin mo yung jeps house of gadget. Online store siya pero ang pickup dun sa robinsons forum. Pwede din naman mag walk-in kaso di sure kung may stock sila sa store nung item mo. Halos hidalgo din ang price nila kaya ok na. I recommend hidalgo pero kung gusto mo less hassle at aircon(haha) i recommnd jeps.
Eto yung sa TPC rin, diba?