Is secret shopper or any online companies to work for legit or a scam?

I am searching for a new job for personal reasons and sent out lost of resumes for customer service jobs, because I have that experience. However I am getting emails for secret shopper or a href="http://forfree.herobo.com/wah/" rel="nofollow"http://forfree.herobo.com/wah//a something along these lines. I don't know if I should respond or is it a scam? Anyone done this?