Seems my programmers think this way:
The person wouldn't have known about the site with out the first guy's link. So, maybe, it was only based upon the fact that he found the site in the first place, that he though to search for it again.
I disagree with this though, and think it's more fair that the second person gets the sale, just because its closer in time to the purchase, and therefore more directly related. Neither of them really "closed" the sale, I like to think we did that.
