I signed up with mysteryshopper.com.au and performed two assignments in good faith only to never be paid for them as promised. I filed a support incident with them and was never contacted regarding it yet they marked it as "resolved". They looked legitimate at first but then the assignments started to "require" purchases that had no bearing on the service evaluation performed.
Don't fall for anything connected to Bean Marketing in Sydney!
I have also contacted the Mystery Shopper Providers Association regarding this as they promote legitimate mystery shopping worldwide. They can be found at www.mysteryshop.org