I've found that to be true myself. Every time I've seen a knife at a site with a significantly lower price than all the competition, that knife was not in stock but on "back order".
Not always true.
Take The Cutlery Shoppe: nobody beats Jeff on Spyderco prices and yet his site is almost always up-to-date. And he only charges you when he is about to ship.
Same with KnifeWorks, very few dealers beat them on Benchmade prices, and yet they are just awesome to deal with.
Now take eBay: SpydieGuys were among the priciest, and they sucked to the point of qualifying for my hall of shame.
On the other hand, Poor Fish has excellent prices and offer great service.