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Craigslist also does not take a cut of the price of the item -- it is solely a marketplace.

They also cooperate with authorities.

Does the phone company monitor your calls to make sure you aren't scamming the pizza-guy? And can you imagine the delay as you had to go back and forth with Craigslist on what constitutes proof you own a ten-year-old bike, or whatever.

How large should my obligation be, to make sure you aren't breaking the law?

If you think there's a Craigslist seller selling illegal goods, arrest them. Nice and easy.

If you think someone's violating the terms of their lease, tell their landlord.

Better answer: CL clearly indicate what activities and items are prohibited, and provide means for users to report these.

The system's far less than perfect, but it does exist and is explicitly supported.