Beyond what the guys have said, if you are buying 2nd hand, do look out for tell-tale signs like claims bodies/lenses are under warranty but do not have receipt to proof. Item is registered online (screen shot provided) etc etc.
Nikon for one requires the receipt to verify date of purchase. For Canon, there are differing feedbacks where Canon only checks their database if registered online while some will need the receipt. I went to Canon twice to service my body and an ixus. Both times they asked for the receipt although they were registered online.
In all fairness, most 2nd hand items I have bought from are friendly and honest members while some are less desirable. Just be careful when buying 2nd hand.