Today only KEH is running a Nikon sale: 8% off on cameras, 12% off on lenses and 20% off on accessories when you use coupon code NIKON11E during checkout. The Nikon section at KEH can be found here.
All used gear sold by KEH comes with a 180-day warranty. KEH is an authorized Nikon dealer. You can also sell your existing camera gear to KEH (any gear, not only Nikon – see this page for more details).