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Most major networks have the ability to pass a special parameter (SID at CJ, bfinfo at BeFree, u1 at Linkshare) that will get reported when there's a sale. You can use that to identify which keyword got clicked, which should give you some indication of which products are getting purchased. For instance:
At CJ you need to click on the "Show Advanced Options" when getting links.
Can I add SID's "on the fly" or do I need to go into CJ and define each one first?
This ShopperID thing seems like it could be pretty powerful and allow you to track sales in a number of different ways.
Just found this which answers pretty much all my questions :)