True. As a matter of fact I was thinking about this the other day (when I updated the paris parallax) but because no-one ever said anything I thought that numbering the layers was not something important. I'll re-organize them and re-upload files tomorrow.
The assets are free because it is something I am doing on the side (amateur level) and I don't want to either have pressure or a "customer/contractor" relationship (with deadline, expectations and so on). I do what I do, when I can.
Eventually (and I am talking about maybe a year or 2 down the line, when (and if) I get much more content on that page) I might reconsider this position and work longer on the layers to sell them but so far they'll remain free.
In any case, thank you for your comment and suggestion. It's nice to see when people appreciate the work.