It sounds to me like you are looking at a genuine Canon, but what is called Grey Market lens. These are genuine product that are imported outside of Canon's official distribution network and sold through 3rd parties (i.e. not authorized dealers).
While they are most likely genuine units, they are not covered by Canon warranty and may not be serviced or supported by Canon locally - which is why the vendor responded that they cover the warranty of this themselves. There is risk: on one side you should be getting the unit at a much lower cost, on the counter consideration you need to be happy that the lens will either be fine through its support life, or that it will be reliably serviced using the vendor's resources - so you pay your money and take your chances, so to speak.
While we cannot encourage you to purchase outside the official channels, if you do decide to consider going this way, then you should look up the reputation of the vendor on the site on which they sell to see how much they have sold and what their satisfaction level is, any comments on support.
cheers, TREVOR
"The Amount of Misery expands to fill the space available"
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
