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
The regular Canon USA new lens is $649Canon EF 70-300mm f/4-5.6 IS USM for $649
at AdoramaCanon EF 70-300mm f/4-5.6 IS USM for $649
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.