I am currently writing an interal advertising system for a company client's website, where the adverts will only be seen by internal users, and all transactions take place internally to the site (i.e. the adverts are for member-only content available on the site).

Does anyone have any recommendations as to the best way to track the conversion rate of these adverts (i.e. views:clicks:sales)?

EDIT

I'm not looking for a 'Why don't you use google analystics'-type answer, I'm looking into possible architecture outlines, i.e. a 'why don't use store a guid in a cache temporarily and see if it ties to the advert' kind of answer.

/EDIT

In a previous job I did something based on an internal cache, which simply did view:click tracking, however the addition of the sales rate makes this task more complex, especially if we take into account the idea that someone may click through to an advert and not purchase immediately.

Cheers, Ed

(N.B. I'm leaving this purposely vague in order to (hopefully) get some answers that provide ideas I've yet to have thought of by coming at the problem from a different angle)