I've seen great results creating unique content on the same domain, such as product reviews, which then link through to the item. With the Google attack on article submission sites and their new frenzy against anyone putting a link in a post to another site, this is more vital than ever.
It's also a great way to boost your position against larger competitors. Amazon might have a lot of products and a lot of traffic, but creating a "*Product name* Review" on your own site can gain you a higher rank than them, simply because they don't have the manpower to write a long review for every product.
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