>>How were they to know the ad wasn’t valid? <<
How were they to know it WAS?
If you passed a house in a neighborhood that had a sign up that said “take what you want. Everything is free.” but there was nobody there, would you?
Would you have a legal case if the cops caught you? Would the local Ace Hardware which supplied the sign material be responsible?
So you're saying that no one should assume any ad placed anywhere is valid? What about the responsibility of the ones placing the ad? How about those who are making money off the site?
If you passed a house in a neighborhood that had a sign up that said “take what you want. Everything is free.”
Not quite the same thing. The operators of Craig's list are making money off running the site. One could easily assume the ads placed are being vetted.