As far as Amazon goes, the percentage they take varies with the category. However, the decision on who to use for fulfillment is a separate one from who to use for a storefront. If you are going to automate fulfillment, I do recommend Amazon. Their price might not be the best, but I think their product is pretty tough to beat.
If you get to the point where Amazon wants to buy your product wholesale and sell it themselves, my experience is that they are pretty tough to work with. I would recommend selling your product on their site yourself, instead.
I wrote the Python modules (as part of the Boto library) for Amazon's Flexible Payment Service and Marketplace Web Services, so I'm a little biased.
