But how do we know killer would get things done correctly?
It's a question of trust, if you don't trust me then don't place an order. Revealing information on finished jobs might be embarassing for the customer, it's common in this business to not talk about where/what jobs where done. It's just an offer...take it or leave it.
Also, why do you say it is legal?
Getting you in contact with someone to kill an annoying, cheating or robbing rat is not illegal. I do it for justice.