First of all a 50% cut is high even by agency standards.
Second, I've never done agency, but I understand the lure of it all seeming easier as the agency supposedly take care of everything. The truth is no-one can look after your safety and security, not to mention screening like YOU can. The majority of agency do not screeen, they don't care two hoots about you as long as you make money for them.
I'm not saying this to scare you, but it's the truth in the majority of cases. And having an ad on Craigslist just doesn't sound like a professional way for an agency to be promoting themselves. You don't know the driver from Adam, or how safe you will be with him.
Honestly if you have this many doubts, I think you are answering your own questions. The other thing is agencies don't employ you, you employ them and if you feel this uncomfortable you do not have to go ahead with this.
Is there any reason, having read thro this site, that you can't put up a profile on Adultwork and start this way. I appreciate it can seem daunting setting up on your own, but there's loads of advice for Independents on here, and we are here for you, if you have any additional questions.