I've got lots! But I think the one that most appeals to libertarians is pointing out the obvious public choice problems. Markets -- including markets in self selling -- only work if the societal institutions around them are good. But societal institutions are made by the people in society, and in societies with slaves they tend to be made by slave holders. And that pretty much never results in good public policy. Here in Texas back in the 1870's, convict labor could be rented out to help defray the cost of incarceration. Which is no,t necessarily a terrible idea. But, of course, it gave the politically powerful class -- business owners -- a huge incentive to screw over the politically powerless class -- the newly freed african american population. Suddenly the law was changed so that a black man could be arrested and given a good sized sentence for just about anything ("loitering" being the great example), they were in fact arrested, in droves, and we had a new sort of slavery.
That's really common. Here was a trick in ancient Rome the rich people tried: go to war, laboring class has to go fight, laboring class can't tend to crops, laboring class gets in debt to you, laboring class can't pay debt, you get their land and themselves as slaves (early roman law was big on debt bondage, I think that got relaxed). So on the one hand, maybe there was a rational decision to take on a debt, knowing failure to pay could result in slavery. But on the other hand, the only reason the poor dude was in that position in the first place was that the rich dudes launched a war!
So yeah, I am not convinced you could ever structure a society where 1. Labor can be coerced, and 2. when it is, it is completely the result of free market forces, with no other nudges from public policy getting people to sell themselves. If you want a model, try this: we've got a rich guy who wants to abuse the slave system and a poor guy who doesn't. Then the rich guy buys the poor guy. Now, in effect, we've got two people who want to abuse the slave system, since the political voice and power of the slave now belongs to the master. Rinse and repeat and eventually the laws and customs around self-selling will take on a coercive, brutal nature.
I don't know if that model is right, I just know that, historically, coerced labor tends to be brutal, ugly, and not fair, even if your definition of "fair" is extremely libertarian.
Which raises the question, are our society's two big forms of coerced labor (convict labor and the draft) good ideas? My answer: probably not! Exhibit A: we don't have a draft anymore and have sharply limited how we use convict labor, because both systems were unpopular and cruel.