... I find it too hard to keep track of sales there to tell with my own files.
I nominated whatever my quota was back in 2011 when I returned from exclusivity. I haven't updated anything since (more uploads means more slots) in part because it's too hard to see what's sold in what numbers. 123rf doesn't sell enough to make it worth spending any time trying to figure this out.
A real test would be to nominate some of your worst files - things you think in retrospect no one in their right mind would buy. If those sell, then you know this promotion works. If it's just your bestsellers you nominate, who's to say they wouldn't be selling well because of the image, not your artificial boost in the rankings