The work was already performed. Instead of offering a money back guarantee, a service business can provide a guarantee to solve the customer's problem. For example, our plumber in the example above can guarantee to come back without charge as often as necessary to stop the leak. A landscaper can replace without charge any plants that don't survive for at least 6 months. A sales consultant can continue working without charge until the promised sales results are achieved.
Nobody can guarantee someone else's success in operating a business.
But, there are some things you can do to lower the buyer's risk. However, you can lower the business risk for your new distributors by offering support in sales and recruiting. Recruit their first few distributors. Conduct 3-way conference calls to help persuade their prospects to sign up. Any sales or recruiting support you offer reduces the business risk and convinces more people to sign up under you.