Ask her terms up front

You shrug, and ask for what she wants as payment. "Not that I'm guaranteeing I'll consider it a fair trade," you add hastily. For some reason, you feel like you'll have to be extra careful with this woman.

"Of course," she says, smiling. "And I think you'll find my terms reasonable. You may have the book in exchange for -"

"Let me guess. My soul?"

"What would I want with that? No, in exchange for your body. I want you to have sex with me. Willingly, without hesitation or inhibition. And I want it right now."

You blink with surprise. This woman is gorgeous! What's she doing soliciting sex from strangers? "What's the catch?" you ask.

"I don't know what you mean."

"I just can't believe that all you want is a roll in the hay. Come on, if you picked me up in a bar, I'd let you have it for free. Why do you want it as payment for something?"

"Explainations are not part of the bargain," she says simply. "You have my price. Are you willing, or not?"