Personally i feel the whole presell thing might not always work conversion wise, unless after the presell on your site the link actually inserts the product in the shopping basket by prefilling the basket or taking the user directly to the checkout. I have yet to come across many programs which prefill the basket after we have presold them on our sites or ppc activity...
...otherwise in effect the customer has to be presold twice?! Once on your site & once again on the merchant site..I suppose what I am rambling on about is that affiliate links should have the ability to take customers to the checkout and/or shopping basket....for improved CR
This whole concept of getting search visitors to your site first, preselling, and then sending them to the merchant is not new... it's called "organic search engine marketing". You guys ought to try it sometime. :-)
Seriously though, there's gonna be a whole lot of affiliates needing to brush up on their preselling skills if Google does what it seems like they're gonna do.
Done the right way, you don't end up "selling" the visitor twice at all. The fine art of preselling is MUCH different from selling.