I doubt it, and the reputable stores are selling the Canon 70-300 for over $600. Remember that there are all sorts of rip-off stores advertising very low priced that are just "bait and switch".

Anyone advertising a significantly lower price than you will find at Adorama, B&H and Amazon is very unlikely to be legitimate

However there is a deal right now on REFURBISHED Canon 70-300 lenses. For example:

REFURBISHED Canon EF 70-300mm f/4-5.6 IS USM for $392 at Adorama

The regular Canon USA new lens is $649

Canon EF 70-300mm f/4-5.6 IS USM for $649 at Adorama

Canon EF 70-300mm f/4-5.6 IS USM for $649 at B&H

The refurbished lens is an excellent deal. I've bought quite a lot of refurbished items from Canon and they've all been like new. $392 is a very good price on an excellent lens. I wouldn't hesitate to buy one if I was interested in that lens.

I don't know where they are getting those lenses. The sellers certainly aren't authorized Canon dealers. I'm pretty sure they can't be getting them from Canon and selling them at that price because Canon have a MAP policy that would prevent them from posting those prices.

Beats me where they are coming from or if they are genuine Canon parts. Could be I guess.

There seem to be a bunch of them out there. Maybe they fell off the back of a truck....

Cheap Canon EF 70-300/4-5.6 IS USM