Sorry but I think you are all incorrect.
No where do I find in Canon's warranty that it will not warranty an item from an unauthorized reseller.
There are benifits of buying an item form an authorized seller, but it doesn't have anything to do with the enforcment of the warranty;
An quote from the 5D III's warranty;
"Canon U.S.A., Inc. and Canon Canada Inc. (collectively "Canon") warrant to the original end-user purchaser, when delivered in new condition in its original container"
A reseller that is not authorized is not the end user, and the warranty does not end or start with him. He would not meet the legal definition of the end user.
Here is the warranty link, I am at work now so couldn't go over it with a fine tooth comb but I find nothing that limits this;http://www.usa.canon.com/cusa/support/consumer/eos_slr_camera_systems/eos_digital_slr_cameras/eos_5d_mark_iii#ServiceAndSupport
If there is other information about Canon not honoring a warranty from an unauthorized reseller then please provide it. I do mean printed material released from Canon, not forum hear say.