You can only sell to a customer once if you are in a marketing program that offers one-time sales products. Take ebooks, for example. They are fantastic, but once you sell it to someone, it is over. They will not repurchase products are those that people often use. Additionally, anything that is regularly used eventually runs out. Additionally, they will need to order more after it runs out. For example, cleaning supplies, phone service, internet, medical supplies, and food supplies, to mention a few.
