I think it is a little difficult to screen for underage or trafficking, because it is not like the people placing the ads outright say that is what is being offered. So how would anyone know unless they actually see the SP in question.
These adds are public, which means that police can read them as well. If it was so obvious that someone was underage or trafficked, what is to stop the police from simply answering the add and doing something about it? The fact that they are not suggests to me that anecdotal accounts say that these issues are widespread, but they are actually not that common. If you look the numbers during one of their regular nationwide sweeps supposedly targeting underage prostitution, the numbers of minors they actually find is usually less than a hundred, from an operation conducted over a number of days with thousands of law enforcement officers taking part. How hard can it be to find these girls? The numbers actually found in these sweeps suggests that there simply are not that many of them out there.
This campaign against Backpage is just being used as a smokescreen to take down the visible aspects of regular prostitution without actually doing anything about it. The police and activists will pat each others backs and congratulate themselves on a job well done, but the ladies are simply going to find a different method to connect with clients, as they have done since money was first invented.