Yes you can buy somoene else his slaves, you do not have to be a slaver for it.
The problem however, is that one of you will have to go first. since you have to talk to the slave and tell him that he has a new master. so you will have to give the caps first. then the slaver will give you the slave. Slaves are really usseful, they can carry allot of items, fight and most importantly, they can run. Making them far better than Brahmins. Slaves are also able to mine just fine.