I guess it all comes down to how 'intent' is interpreted. If someone is offering prostitution on craigslist, but craigslist doesn't catch it, is craigslist liab

I guess it all comes down to how 'intent' is interpreted. If someone is offering prostitution on craigslist, but craigslist doesn't catch it, is craigslist liable because they "didn't do enough" to prevent it from happening? That was probably a silly example because I think craigslist is fairly active in policing themselves, but there are lots of websites that are not as stringent, because of resourcing, etc. Are there cases where the fine line between unintentional negligence (if it can be proven at all?) and intent is blurred?