Unfortunately, there are a lot of rip-off artists working the Internet, at least in the USA. many of these are simply store fronts with just an address but, no merchandise.
They will advertise gear at a very low price and after the poor soul pays for the gear, they will try to sell the accessories which Canon supplies with the lens at crazy prices. This boosts the price of the gear well above the normal retail price and that is how the scammer makes his profit. If the sucker doesn't want to pay the price, they say the item is not in stock and the customer has the Devil's own time getting his money back. It is bad to pay with debit cards and crazy to pay with money orders of checks. Because the seller has your cash and you have to get it back from him. At least with a credit card, you have some recourse.
The newest scam I have heard of regarding Canon 24-70mm f/2.8L lenses is claiming that the rash of poor focusing has resulted from lenses produced in Malaysia or Taiwan or Thailand, and that the sellers lenses are more expensive because they are manufactured in Japan. What hogwash!
For some reason, these gypsy scammers often work out of Brooklyn, New York, When the district attorney closes one down, it just opens under a different name. I had some problems with a Brooklyn reseller when I first bought my Canon 10D many years ago and never purchased from one again.
I am sure there are honest resellers in Brooklyn and in Eastern Europe, JUST AS I AM CERTAIN THAT A FLOCK OF PIGS JUST FLEW OVER MY HOUSE!
For U.S. buyers, check http://www.resellerratings.com/ for comments on Internet sellers. I am sure that there are equivalent sites in the U.K., New Zealand and Australia.