I'm looking to buy used Photo equipment, Canon only as I don't know Nikon,
my suggestion - get a used canon camera in a shop that offers 3-6 months warranty.
you see, the main problem with buying used cameras is that there is a certain life span for shutters.. for example 5d mark1 is around 100'000, 5d mark2 150'000 (plus/minus) etc..
I had to learn this mistake on my own :/
good luck,
H
p.s. if you are in the UK - check Park Cameras