Of course there's a market - people barter images to get access to more images.

This is one justification used to stop a paedophile from working with all children even if their particular interest is strictly not all children. If the paedophile is only interested in boys under 11 you still need to be careful when he's working with girls over 13 because he can use his access to those girls to get photographs which he then uses to get access to photographs of his target group.

Devil's advocate: if we accept that, then in effect the ban on child pornography could either be massively proliferating child abuse, or massively proliferating the collection of child porn. Maybe both.