The method of distribution/sale has everything to do with the content of what is being sold. At one end, softcore (Playboy-type) is sold just about everywhere; at the other end, extreme hardcore is available at adult bookstores/mail order. Kiddie porn is underground.
What am I telling you that you don't know? What is the point of your question -- would you please make one? I'm getting just a little frustrated here with your feigned ignorance (I hope it's feigned).
I am still waiting for you to show how opening additional methods of LEGAL distribution of porn will lead to an increase in ILLEGAL child porn.