Kiddushin 42: When Is an Agent Not an Agent? (For Sin)

How do we know shelichut (appointing an agent) works, with some pushback as to whether it should work at all. Plus, the idea of benefitting another person, and whether action taken on another's behalf is necessarily shelichut. Also, who is responsible for damages in the process of shelichut - the agent or the one who appoints an agent? Note the important principle that there's no agent for a sin, because the agent himself needs to heed God's instructions over and above those of the person who would appoint the shaliach.