From AMLopedia
Customers, Customers! was a song/phrase coined by Marcus around 1999. His family was having a garage sale, and the three boys were sitting on the trampoline devising ways to get more customers to shop at the garage sale. Marcus suddenly came up with the genius idea of lifting his shirt up, flashing the customers, and singing "Customers, Customers!" This was forever sung whenever garage sales were mentioned in the future.