Blind Trust

My photography class teacher once told us that if we wanted to feel a place, we had to go to the public market. We are up in the mountains of Baguio and today was market day for me. Passing the back alley, I saw this man, he cannot see. I stayed awhile to observe how he does his business and went about selling his goods. I noticed a man pick up a bag of beans and gives him the exact amount of 20 pesos. How would he know if the money exchanged was correct? He conducts his trade out of blind trust.