When I first came to Korea in 1977, I was fascinated by the new culture and all the culturally related articles in Korean shops. I would often go into the souvenir, antique, and clothing shops in front of Camp Humphrey just to browse. If I went in the morning, Korean shopkeepers would often tell me that I was their first customer of the day and could, therefore, get a good deal from them. I did not really really expect much of a deal by just being their first customer and would often leave the shops without buying anything. At the time, I did not know that my leaving the store without buying anything portended a bad day of business for the shopkeeper, which was probably one reason that they rarely wished me a good day on my leaving.
Many Koreans used to believe that if they could sell to the first customer to come into their shops, they would have a good business day. If the first customer left without buying anything, they would have a bad day. The Korean expression used to refer to this first sale of the day is 마수를 걸다, which means "to make the first sale of the day." You can also say 마수걸이하다.
Supposedly, the first customer of the day could get the best deal because shopkeepers would be more willing to sacrifice their profit on the first sale to ensure good sales for the rest of the day.