MicrostockGroup

I'm new here and not overly involved in Microstock, but I've noticed a strange trend at Fotolia. When I buy some photos for one of my blogs... usually within 24 hours I also get a sale or two on one of my files. I have 150 clips listed with Fotolia and very few photos. This has happened for the last 7 files that I purchased. It's actually a good deal since I buy extra small and then a video clip is purchased and I get $3-14 commission.

So is this some special algorithm that Fotolia uses?