How does a real estate lock box work?

Hi, I am selling my home by owner. I have an offer to list it on the MLS for $179 and still retain my right to sell by owner. I am thinking of offering agents 2.5% or 3% commission for bringing me a buyer. I see that I can buy a digital lock box on the internet for only about $40. As I am not home most antytime, I would like them to be able to gain access. I know the lock box would have a code and I would put my key in there, but how do I get the code to the agent? How do I know they are an agent and not someone robbing my home? Can anyone offer any information that could help me out? Thanks