onlineleben - 9:42 am on Dec 10, 2012 (gmt 0)

Don't know how to program this (should be a few simple lines of php), but in my thinking it should work like this:
a) visitor comes to your site
b) store the search phrase and referer in a cookie
c) visitor goes to merchant site and buys
d) visitor returns to your site's thank-you-page
e) read out the cookie and store the cookie data in a text file