3. That will be difficulty.
Technically speaking, while mailbox is not a part of the property (i.e. you don't own it), still it is within the owner's control (subject to relevant ordinances). Unless prior arrangement has been made, the previous owner no longer has rights to open the mailbox of the property once the sale is completed. The previous owner's action has constitute civil trespass in the civil law perspective.
Although the previous owner was wrong, the problem here is you have to prove you were injured by the action of the previous owner, i.e. you have to show how the previous owner's action caused your damage. In this case, it is extremely difficult, as:
a. It is unlikely that you have mails to the new property, given the property was just sold recently.
b. You can't prove that any of your mails have been taken away.
On that basis, the best you can do is $1 nominal damage.
It is simply not worth the effort.