That might be okay. Another possible real-life alternative (not an attorney, so not sure if this would work) would be for the slaves to auction off their clothing rather than themselves. They would then be left naked and could choose to do whatever the owner of their clothing demanded to try to earn their clothing back. Of course, they could also refuse to submit to those demands, but that would mean having to walk home totally naked.
another option would be that there was no money involved. what if the bidders were given a certain number of 'points' to use, with them bidding up each other in points for the right to mess around with the guy they like the best? again, no actual sex would be required, but the video would give the impression that it was going to happen?