I've always associated the sissy maid fantasy as something one would act out with a dominatrix, either a pro or their SO. Therefore I suspect the ad you saw was a for a pro domme.
Like others have said, you'd want to meet them first in a public place and decide how you want to play it from there. If the person claimed to be a professional dominatrix they should have no problem with that as they will want to check out a prospective client in a safe environment first. Another clue that they are probably legit is that they will keep conversation and email all business and not sensual or arousing in content. You have to pay for that. I'd expect such a session to cost $100 - $300 per hour depending on where the session is done and how detailed you want it.
Christine
