I'm assuming you have some kind of a server-side tracking solution which accepts tracker pixels.
If not, you can add a random (and unique) number for each visitor's requests and process your server logs.
Replace st3goudkov.txt with your pixel. It does not really have to be a pixel.
Two request parameters are vScr and tTime.
vScr is the percentage of the pages scrolled down. It's better than pixels, because it compensates for difference in resolution and the size of the browser window.
tTime is the time in seconds.
This script will make calls in the backround every 0.7 seconds if a visitor has scrolled down further than in the prevous call.
Normally, those requests will have teh same cookies as regular requets of the same browser, so your server-side tracking should be able to identify the visitor.
Use it at your own risk.
To activate, add onLoad="startTk()" to the body tag.
And to check your current position on your sales page, put this in the address bar and hit enter (all on one line):
If you have any questions then drop me a line at k AT goudkov DOT com
Konstantin Goudkov,
www.goudkov.com