Stolen items are fenced on eBay. The payment processors don't have an issue with actual crimes like theft but want to impose a moral code on legal behavior.
Like how would you distinguish between a stolen item and a used item without requiring people to keep every receipt for anything they ever buy? This is not an endorsement of their anti-porn policy, just observing that that it's easy for them to implement that in contrast to your theft example.
User-traded goods on e-Bay are also much harder to verify facts about than goods from a brick-and-mortar shop. Doesn't seem to prevent e-Bay from operating.