The reason I think it's reasonable to limit your right to sell yourself as a slave is because if it wasn't limited then a lot of people in very precarious economic situations would sell themselves and there would be a very real interest in creating those situations to force more people into slavery. You can see that with the usurious interest charged by payday loan companies. Usury is also usually illegal in civilized societies for a similar reason.
Similarly you shouldn't be allowed to abrogate your right to privacy because it creates an incentive to force other people to do so as well.
The reason I think it's reasonable to limit your right to sell yourself as a slave is because if it wasn't limited then a lot of people in very precarious economic situations would sell themselves and there would be a very real interest in creating those situations to force more people into slavery. You can see that with the usurious interest charged by payday loan companies. Usury is also usually illegal in civilized societies for a similar reason.
Similarly you shouldn't be allowed to abrogate your right to privacy because it creates an incentive to force other people to do so as well.