Funny, I was just trying to figure this out - especially with posting our products on Amazon. How great would it be not to have to rewrite our content for amazon. But, I came across this on the google webmaster site:
In most cases Google does a good job of handling this type of duplication. However, you may also want to consider content that's being duplicated across domains. In particular, deciding to build a site whose purpose inherently involves content duplication is something you should think twice about if your business model is going to rely on search traffic, unless you can add a lot of additional value for users. For example, we sometimes hear from Amazon.com affiliates who are having a hard time ranking for content that originates solely from Amazon. Is this because Google wants to stop them from trying to sell Everyone Poops? No; it's because how the heck are they going to outrank Amazon if they're providing the exact same listing? Amazon has a lot of online business authority (most likely more than a typical Amazon affiliate site does), and the average Google search user probably wants the original information on Amazon, unless the affiliate site has added a significant amount of additional value.
The point is well taken - if you have the exact text as shopping.com or amazon.com, you're going to be ranked below them. That simple.
The whole article: