There is no single method for creating and retaining such documentation. Different MLMs employ a variety of approaches to demonstrate that their product is sold to retail customers, including collecting retail sales receipts created by participants; having retail customers buy product directly from the company, rather than from a participant’s inventory; and having product users sign up with the company as customers who are not participating in a business opportunity. Other MLMs use other approaches or a combination of approaches.
Often the only way to make these sales is to recruit people under you (making commission off their starter kits) or to buy products yourself. Otherwise you’re left trying to sell your products to friends, family, mums at the school gates, and anyone you come into contact with (one of the reasons why some of the more pushy/desperate MLM reps get a bad reputation).