I'm not understanding what happened. First you said you contacted the "ad lister" who showed you the place and then you're talking about a real estate agent. Typically if YOU contacted THEM, you would be working directly with the listing agent or why would she have placed an ad on Craigslist? She should be being paid by the landlord.
Also, I always recommend to people that they fully read whatever they are signing. If a document said you were responsible for 25% of a first month's rent, even if she told you you were not responsible, why would you sign that?
Then you said you never contacted the lady for her services, but you said at the top of your post that you contacted the "ad lister." I guess I just don't understand at all what happened here.