If I understand correctly you are asking how to answer the question on application forms that ask, "Why you believe you would be a valuable shopper?" As Mantis said, "tell the truth."
Why do you think you would be a good mystery shopper? Is it because you are observant, you pay attention to detail, you have a good memory, and you are objective? Pick out your personal qualities that you think will make you a good shopper. Depending on the space provided, give them examples of how your previous work experience required similar skills.
Hope this helps