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We are currently in the process of reviewing our site on a number of fronts, and part of that involves adding quality content to our product category pages where necessary. Our practice so far has been to add relevant content, and in this content externally link to a couple of relevant, related sites for that category (avoiding the use of keywords we are trying to rank for). Should we be doing this (externally linking)? And, if so, should these external links be NoFollow? So far, we have seen mixed results for our efforts.

I'm fairly new to the whole SEO and link building scene so I'm trying to get a handle of high value, low value, and negative value links.  For example, I had a discussion with my boss on the idea of affiliating our e-commerce site with Amazon.   From a business standpoint, it'd probably increase sales. However, I was wondering if  those links would have value for search engines? Thanks