You might reply that it might be difficult to prove that money changed hands, particularly in this brave new Bitcoin world. Fine. Let's say that possession of images that are known to have been sold to anyone at any time constitutes prima facie circumstantial evidence that the person possessing the images paid for them.
Another objection might be that this law might not suffice for catching cabals of child pornographers who create and share images, but without money changing hands. Fine -- let's generalize the criterion to include any exchange of value, including in-kind exchanges. So trading images without monetary payment would still be illegal. (Of course, creating the images would remain highly illegal.)
The outcome would be that the provenance of the image would matter. Possessing an image taken by oneself, or by a friend, possibly by the subject -- all these would be legal, so long as no other crime was committed in the creation of the image or in obtaining it. But downloading material that was being circulated on the Net would still be very dangerous -- the probability of picking up an image that someone had paid for (or bartered for) sometime would soon approach 100%.
Let's call such images "tainted images". The provision that possession of a tainted image constitutes prima facie (i.e., rebuttable) evidence is important. It means that if you visit a Web site that pops up images of child porn, you haven't thereby committed a crime, so long as forensic examination of your computer supports your contention that this was accidental and not part of a pattern of behavior (e.g. there are only a couple of such images and they're only in your browser cache). Well, IANAL and I probably don't know exactly how the law should be phrased, but I think it should be along these lines.