I like when vanilla girls reply to ads. Whether they're college students, or older women with full-time 9-5 jobs. Your scenario I also like. It's cool seeing them put on the spot. I'd never be brave enough to go up to a girl and ask though.
Someone I graduated high school with, during her freshman year in college while she was reading a book in the park, had another girl offer her $100 to play with her belly/bellybutton for 10 minutes. The girl played with it, while the girls boyfriend filmed it. She let her do it while she kept on reading. And not only did she let the girl do it, she let her go longer. And she got more money than originally offered because of it. This was a story she told a group of us years ago when some of us got together for a small gathering to catch up. I didn't say much, but inside I was like, "Man, I wish I could've seen that".