For me the answer is obviously YES, although I'm pretty sure there are going to be some people who disagree.
Even if she agrees to sell sex, the man could sexually assault her in a way that is too violent or aggressive for her liking. She could be scared and alarmed and yell at him to stop but the guy could keep going.
So say he responds to an ad on the backpages, they agree to meet at a place, say $200 an hour for sex...Does that mean the guy is allowed to fuck her any way he wants to? Does he have to exhibit some restraint or is the girl technically his sex slave for the next hour?