Ebay has the same kind of rules, prohibiting "sexually oriented materials" and allowing nudity "only in limited situations". (https://www.ebay.com/help/policies/prohibited-restricted-ite...)
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.
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.