This is exactly the reason why independent contractors being hired as mystery shoppers does not work.
I love the writer's on-the-job training. But the more shops you do, the better you get at it—and the more likely that shop providers will call and offer you first pick.
What about the client who paid for the shopper to get that on-the-job training?
You may be asked to follow a script, i.e., become a good liar. For example, my daughter was told to go to a pet-food store and say she had two dogs. At the time, she didn’t own so much as a goldfish. Really? If the shopper had some sort of practical experience, won't they be a better shopper?