There are legitimate Secret shopper sites - they however pay very modest amounts - usually $5 -$10 per shop. They also have very strict rules of conduct and online courses that you need to take to be qualified. Corporate Research is one such site.
If you are contacted by
ANY company soliciting employment here are things to ask (I know - I too was scammed)
1) Actual address of the company (and if this doesn't feel right, the owners home address
2) Company phone number -
not just a fax number
3) Owner and/or CEO's full legal name
Before you think about doing business with them - check them out with the BBB, FBI and any other company they may refer you to. If your intuition tells you that this is bad - listen to it and run in the opposite direction.
But
NEVER fall into a situation where they either send you a huge check or their clients do, have you wire money back after taking a commission for your time -
this is a scam!