What are some of the sites available for buying used lenses for the Nikon D-90 camera body? Do any specialize in Nikon or have enough supply to deeply cover Nikon lenses?
One I always hear about is KEH. They are in Georgia.
Apart from KEH that MikeW already mentioned, take a look at Grays of Westminster. I found their collection pretty good and complete. It is an exclusive Nikon shop. The shop is in UK and referred to as "The High Church of Nikon". Their website, unlike other online store websites, is very well organised.
While looking for a used lens I came across fredmiranda.com and ended up making a purchase through them at a good price.