I found some of mine, which means that I have duplicates on Fotolia, now!
500px is selling the photos in a premium category with $250 large and $120 small, compared with 12 credits or 6 credits, same thing sold by Fotolia. They have the enhanced license option selected with no option to un-check it. Since my ELs are $100 on Fotolia, I don't mind. Maybe some clients can be attracted by the concentration of good photos in this collection and pay a premium without searching further.
But the keywords are very poorly selected, missing the essential ones, therefore chances to fall on these photos through regular searches are minimal.
Btw, they give credit now e.g. "original author"/500px.