They've been around a while (don't remember, but years) and the exclusive claim is about the free photos. They call SS the "premium" photos. There is a bar of those results above the free photos (if there are any). I put in a term that had no free results (remodeling) and up popped a new window with SS results on their site (versus just the bar above the free ones).
We have no insight at all as to whether these partnerships are generating any sales, but as all Avopix has is the thumb - everything ends up at SS if you click on a SS thumb - I can't see how they can be a problem.