Digby -- I can see the Amazon Portal selling 1/3 of all goods...however, I have 5 independent sites and also sell through Amazon. So, that statistic may be rather misleading...I don't compete against very much product which is "shipped from and sold by Amazon.com"...but compete against other sellers listed on the Amazon.com framework.
Amazon sales are much slower compared to my own sites -- maybe 3-5%. Order value is typically greater, though, and their rate is set, so my cost per conversion is always the same. Margins are comparable, actually, though I don't get as much control over the shipping as I'd prefer (in regards to their algorithm -- you set a base rate and then an additional charge per pound of product...not algorithm + zone shipping like I do with my own sites).
That being said, my major site dates back to '98 and I've only been live on Amazon since January, so it's not exactly apples to oranges. Also, my products have not been available on Amazon for very long and I suspect most users are not yet aware that you can buy home improvement items through their portal.