If you do buy a used camera, I'd recommend downloading the manual prior to purchase, and then making sure that the "kit" on offer contains all the important items, including the software. (From what I understand, Canon only supply upgrades on the internet to their DPP package which is supplied on CD. Don't know Nikon's policy.)
I was surprised to discover that a new battery charger for my used "pro" model costs ~Ł300, and it's difficult to keep a DSLR running without a charger.

Good luck with your purchase.