I have affiliate relationships with companies that give me access to their datafeed of products and another relationship where a major spares company has created a skinned version of their spares site for me. I've long since made all links to these sites or individual items nofollow and even put a noindex instruction in robots.txt to prevent spidering of my affiliate's database of products. But recently I've started to wonder if this is what I should be doing or not because presumably it means none of these products are ever likely to come up in search engines yet my sites rank highly for related terms. I'm confused as to whether I should be trying to prevent search engines seeing these products which people may be searching for or not.