First of all, you can buy a new one for just over $6,000. This is due to the exchange rate of US dollars in Canada. The lens will have a full North America Canon Warranty, and can be serviced at any US or Canadian service facility.
If you are within driving distance of Canada, you can pick it up. There is no customs duty. Its definitely better than a used one for the same price!! See the note on the CPW page with a link to it.
You did not say if the one you saw was used or refurbished. Canon gives their refurbished lenses a full 1 year warranty as well as a return for any reason within 2 weeks. Many stores do not accept returns of used equipment.
Canon charges $5839, but they are out of stock right now, after offering 15 or 20% off that price for Mothers Day. They will be back in stock before long.
Don't pay higher than the Canon price, there refurbs are usually in condition 10, and have that factory warranty.
Also check CPW or signup for a notification for price drops.
Thanks for the replies. I've been signed up for the CPW notifications for a while. They never have the 300mm 2.8 in stock during the sale events. And lately they don't have it in stock at all. Getting it from Canon refurbed during a sale would be my #1 choice.
I've contemplated the Canada option... All the anecdotal stories I've seen online have mentioned NAFTA (no tariffs) on items under certain amounts but they mentioned still needing a customs broker and long wait periods to get their purchases.
Life is tough when you're cheap and impatient!