[tbp]

The merchants can't actually see the referrering keywords etc, but the networks can. When someone clicks a direct to merchant link, it first goes to the affiliate network, they then redirect on to the merchant. As a result the merchant only sees the refferer as the affiliate network itself.

It's easy enough to do this on your own site. Not only can you build a better landing page than the merchants page, which caters for more of your keyword (which lowers CPC), you can also record all the data you need before you redirect to the affiliate network.

Its a bit more work than direct to merchants, but the benefits outweigh the amount of work you need to do. Your landing page can really do a hard sell, and give a lot more info about the products than the merchants site, which then leads to more conversions.