By giving the workers a simple task, Ripps encourages transparency, where most would prefer romance. So is this exploitation, if the workers aren't fully aware of what they're getting into? Well, firstly: we shouldn't assume that this is the case; are the workers ignorant, or simply unconcerned? Secondly: they are most certainly informed by now. And if they should regret the decision after the transaction, they know to keep from doing business with mister Ripps again, or from deviating too far from their usual trades.
Buy for less, sell for more has always been the model for making money. And selling for more than you're worth (would you pay $80 for those drawings?) is just the business archetype. Ripps put the concept of profit on stage and received a storm of hate from the smart-phone wielding public of infinite bandwidth. He deleted the thread when things got too hot, which is unfortunate. I suppose he felt overwhelmed and intimidated. On the bright side, it's good to see people showing concern for sex workers: caring enough to verbally attack an artist over social media, at least.