OK so you have to differentiate your brand somehow from these big sites. And if they are using your type of products for "loss leaders" (loose money up front but make it up on repeat sales) you may have a bit of a battle on your hands.
Can you go deeper into your niche? Slice up the customers for your market and find a productive subset? I'm thinking something like customers who prefer only-natural, not-tested-on-animals widgets? (You get what I mean)