Some advice i gave to someone else on this forum:
It sounds like Mystery Shopping would fit nicely into your resume based on your career field. I facilitate classes on resume writing for incarcerated individuals and one of the first things I tell them is never underestimate your experiences and never sell yourself short. So, if mystery shopping fits into your career field and can help show off your skills, I think it would be great to use.
Regarding using the word "consultant," I'd say it depends on the company and the report. Some companies, you really are a consultant giving advice on how to improve their practices or customer service. Some, you just fill in what happened with no open-ended questions.
Possible job titles (definitely put Self-Employed as your employer as someone mentioned above):
Customer Experience Evaluator, Analyst, or Consultant
Consumer Experience Evaluator, Analyst, or Consultant
You could even do something such as Customer Experience Evaluator and Consultant with your employer then of course being"Self-Employed"
For your dot points, I'd then gear your activities and responsibilities to customer service type skills.
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For you, make sure your dot points are geared toward skills you need in your career field. Doing analytical work? Talk about analyzing certain aspects etc.