1. How are reparations collected? If you pop my tire, but don't want to pay for a new one, how do I get the money from you?
Insurance would be your best bet. You could also try to steal something of equal value from said person. But if they have the money to pay you, then they may have the money for a better defense than your offense. The threat alone may get you your money. You could also start a campaign to let everyone know about the situation. An AnCap society is based on trust, so damaging evidence of the contrary could really limit that person's future income. Given all the options, I don't think it is in this person's best interest to take those actions.
2. Common Defense and Leeches: It's all fine to think "everyone will chip in because it's in their best interest to have military protection," but there's a Tragedy of the Commons problem there: "I'll avoid chipping in to maximize my own personal wealth and hope that everyone else chips in."
There's another thread going on right now about this -
https://bitcointalksearch.org/topic/m.1178136Basically, if you don't pay your share (to some company), no one is going to protect your property or pay for your damages.